It all depends on several aspects.
- Are you practicing a martial aspect of the taijiquan
- Does your teacher/school teach the martial aspect of the taijiquan
- How much do you practice and what is the main focus of your practice
- Who you practice with
- What else do you do to advance your skill
This is necessarily not a full list, but the main things should be there.
Let me elaborate on these points and then I'll give you an answer.
Are you practicing a martial aspect of the taijiquan
This is a legitimate question as the martial practice of taijiquan is rare. So much so that even some practicioners of taijiquan who confided themselves fairly advanced adepts of the art do not even concisder taijiquan a martial art.
Studying taijiquan as a martial art means that one must always see and practice taijiquan as martial art. Just as the Yang Chenfu once said - you can have enormous civil benefits from practicing taijiquan but you must first understand/achieve the martial aspects of it to get the civil benefits.
Without practicing the martial aspects of taijiquan you can't even start getting the martial proficiency.
Does your teacher/school teach the martial aspect of the taijiquan
Basically the same as previous point, but the onus here is on the teacher. If the teacher does not teach the martial art you really can't learn to fight. What does it mean to teach a martial art is an entirely another topic.
Enough to say that a good teacher can mean a world of difference in the speed of learning. One can try and achieve the same by self-learning but a good teacher can direct your focus and increase your skill level at a dramatical pace.
How much do you practice and what is the main focus of your practice
It does not really matter what martial art do you practice and who is you teacher, if you only practice that one Thursday night after work every two weeks. Practice is the only thing that can get you from no skill to perfection - and all the points in between...
What do you practice is also important. If all you do is form and choreographed application drills, you are not going to be any better in a real fight in a year or even ten years than you were when you started.
You have to practice actual uncooperative and physical fighting to hone the skills you learn from form and tuishou and basic exercises or drills. In addition to adding additional context to your drills and improving your form, it allows to season your skills and overcome your fears as no other practice can.
Who you practice with
You can't get a decent martial skill just by practicing alone. You need the feedback of your classmates just as much as from your teacher. If your practice partners do not want to spar, neither can you. Simple as that.
What else do you do to advance your skill
Let's just agree wit this - unless you are a prodigy, born to the fate of becoming the best fighter on this side of the galaxy, you will have to work for this. No matter how bright you are if you only put in the absolute minimum required from you in the class, you are always destined to be an average. And an average in the fight is just ... adequate at the best. Even less when it comes to taijiquan. Taijiquan (even more so than other internal martial arts) seems at times counterintuitive. You have to let go of your tensions and relax into tha now. It can be hard if you are only learning the basics, but it is essential if you are learning taijiquan.
So how long should it take to gain fighting proficiency in tai chi
If you are properly focusing on the martial training and if you have a good teacher also proficient in the martial aspect of taijiquan. If you have a motivated class full of eager classmates who all share your desire to learn the full taijiquan system instead only bits and pieces and are ready to be engaged in all aspects of the practice. And if you include sparring in your curriculum, I'd say it should not take more than month or two to become adequate fighter.
It does not mean you'll win every match or even most of the matches. Also what you are doing is not necessarily using all the base principles of taijiquan. It simply means you will be able to hold your own in the fight and give as well as receive.
To become a proficient taijiquan fighter will definitely take more time. By proficient taijiquan fighter I mean that in addition to holding your own in a fight you also understand and are able to utilize taijiquan principles and body mechanics. This will certainly take years and can be a lifelong pursuit of perfection.