How long can I grow my hair after the FUE transplantation?

It is best to address your pre operative concerns with your doctor.  If you trust your doctor to perform surgery on your body, you should trust your doctor for these type of questions.  You can cut your hair any way you like and 2900 may cause some thinning look if you have a low donor hair density and keep your hair very short.

Hello, Thank you for your question. After FUE hair transplant, I normally suggest my patients to wait with haircut at least for 1 month. About the hairstyle, you can let your hair keep growing. Once the hair grown back, they will be as natural as your hair. 

If the hair transplant is done correctly and at a high density, then you should have as many styling options as you wish.

You definitely are not required to have a short haircut after FUE. You'll have to shave it down for the procedure itself, but afterward you can grow it out as long as you'd like.
As for your donor area, the typical male can donate 5000+ grafts before noticeable thinning in the donor area. Assuming the hair in your donor area is normal, it is doubtful that 2900 grafts will cause a noticeable difference, but without examining you in person there's no way to guarantee that. If you're unsure you can always get a second opinion.

Thank you for asking the question.

I think maybe  you misunderstood what your Doctor had suggested. You can keep your hair as long as you want you can make a ponytail, if you want after a few months. Hair grow  takes at least 9 to 10 months for a complete result and after that it grows indefinitely for as long as you want them to grow so you are not required to keep it short just you can style it the way you want.

2900 grafts is not a large Transplant at all and it leave no donor side effects at all.

Regards, Dr. Amit Gupta.

Once your procedure is completed, you can grow your hair to any length you wish. Long hair is nice to have and that is what you are paying for