This may be new information to you, or it might be something you already understand. Taiji is done slowly for a very good reason. It's not for meditation, although many Taiji schools teach it that way. The real reason it's done slowly is because you're trying to move your body in a very special way that requires your brain to concentrate on many variables at the same time. This is in order to practice what is known as "internal mechanics".
The practice of internal mechanics requires a high degree of attention to detail and concentration on how you're moving. People who are new to Taiji won't be able to move fast while getting the internal mechanics correctly. They need to move very slowly, because their brains are juggling a lot of different variables. It's very challenging!
Internal mechanics consists of a number of "jin". Jin is a word used to describe "qualities of motion". Each movement in your Taiji form requires a different jin or a combination of two or more jin. There are 8 core jin: p'eng, lu, ji, an, cai, lie, zhou, and k'ao. There are at least a dozen other jin that are taught in Taiji.
For example, in the beginning movement of most forms of Taiji, you're raising your arms up. During this motion, you are practicing something called "p'eng jin". P'eng jin requires you to tighten your tendons so that your bones form a solid connection from your arms to your feet. So if someone were to push down on your arms when you are doing p'eng jin, the force of his push gets channeled from your arms to your feet. It's a little like a bridge or an arch in architectural terms.
In addition to that, during p'eng jin you must maintain a degree of freedom with your joints such that they must remain bent, not locked, and therefore neutral. That allows your entire body to channel the forces against it in a dynamic way, making adjustments quickly and without resistance in real-time. In fighting, the advantage is that you're never over-extended in any way, and so you remain stable, balanced, and able to adapt. Locking your joints in Taiji is generally considered bad form.
There's even more to p'eng jin than that, though. And we haven't even gone past that one jin. There are over 20 more.
What I'm getting at is that during your Taiji form practice, you are imagining people pushing, pulling, grabbing, or striking you. At the same time, you are trying to perform the correct jin. Each jin has a number of parameters that must be maintained in order for the jin to be considered correct to form. So while you're doing your form, you're going down this laundry list of parameters for each jin in your mind. (When you get past the beginner stage after several years, you should know it by feel instead of having to think about it as much.)
As you can see, it's not easy. It's why you're doing it very slowly. It will take a good 10 years of constant attention to detail and practice before you will be able to move more quickly. And even then, it's unlikely you could use it at full fighting speed for quite some time after that. It really depends on the quality of your instruction.
I go over this a little more in my answer here:
Now getting back to your primary question, how long should it take to perform your form? The answer is now a little more clear: It should take as long as is required for you to perform the techniques with the correct jin.
In a competition - which is where you'll more often see this 24 technique form performed - you probably will have a time limit. And so you need to complete the form in that time limit. For example, 5 minutes. But in real life, take as long as you need.
If you're performing it with others in a group, it's probably best to stay in sync with each other so you don't run into each other. In that case, you usually have a leader or a row of leaders that you follow. If you're in the front row, you get to set the pace.
One other thing I need to emphasize. Internal mechanics is the real reason for the existence of Taiji and why it looks the way it does when it is practiced. But it's actually more common to be a student of Taiji and have never even heard of internal mechanics, let alone understand it correctly. That's because most Taiji schools (99%) today do not teach it.
Over the years, there has been increasing emphasis on the meditative, health, mind/body, and spiritual aspects of Taiji. Part of that has to do with certain high profile Taiji instructors who decided, over a hundred years ago, to open up Taiji practice to the general public. Prior to then, it was a closed door practice taught to few people.
When teaching the public Taiji, the emphasis shifted from its martial aspects to all the other aspects. In fact, the martial applications were often thought of as "secret" and reserved for the few who could pay for it. Since then, generations of people have learned it and passed it on to the next generation without ever knowing anything about internal mechanics and the martial applications of Taiji, aside from myths, legends, and theories they might have.
Now, don't let that dissuade you from practicing it as a form of meditation or as a way to improve functional fitness. If that's what you want from it, that's fine. There's a long history of it being taught that way, and they have their own knowledge they pass on. Studies have been done on people who practice Taiji, and they conclude that there are many benefits. So you don't need to know internal mechanics at all to do your form the way you want and for whatever purpose you have.
But the reason I'm mentioning this is because a lot of questions like yours can be answered if you know why it was designed to look the way it does. It all comes down to internal mechanics and the original martial application. Various forms of Taiji have diverged pretty far from its origins, but it's still there.
And if what I told you is new to you, and you do want to pursue a study of internal mechanics and learn the martial applications of Taiji, here's what I recommend doing. First, look for Chen style Taiji teachers near you. Chen style almost always gets taught by having you learn the internal mechanics. So it's more likely that you'll find someone who can teach it to you if you look at Chen style schools. But ask around the various Taiji schools in your area asking who the best teachers are in your area. It doesn't necessarily have to be a Chen style teacher.
If there is nobody teaching it near you, you'll have to look for and sign up to seminars and workshops to learn Chen style. The very first thing you want to start learning is something known as "silk reeling" (chan su jing). There are youtube videos that show it. It's going to introduce you to the 4 basic jin: p'eng, lu, ji, and an. From there, you can begin to apply it in your forms. Learning from video is difficult and imprecise. Which is why you want to get to some Chen style workshops on this subject where you can learn hands-on from a good instructor. They can show you what it feels like, not just what it looks like.
And finally, you could pack your bags and head to Chen Village, in China. There are tour guides that set it up for you. You just have to decide how long you want to stay.
Oh, and you should look into Taiji web forums where you can ask your questions of people who just do Taiji.
Hope that helps.