After how many months can I trim or cut my transplanted hair?

There should be no issue of cutting or trimming your transplanted hair.  Transplanted hair is just like your donor hair.  There is no time limit.

We usually advise our patients who have had follicular unit extraction (FUE) to wait 5-6 days after their procedure before trimming their hair.  If a patient has had a linear extraction procedure ("strip") it is probably safest to wait until after the sutures are removed at 10-12 days.

We generally discuss the care needed after a hair transplant with all our patients.  In some cases we refer them to hair stylist who know how to deal with newly transplanted hair but overall there is not much limitation in cutting or styling after the first few days past a hair transplant.  To be on the safe side wait 7 days and you probably can do anything you want at that point.

If your skin is not too much sensitive you can shave after 3 months, but it is better to leave them grow.
And can cut with scisors after 2 weeks

You can have a good hair cut about 30 days after a hair transplant, or earlier if your hair stylist understands how to be careful with both the donor and recipient area. If your hair stylist understand these nuances, then you can have your hair cut at 2 weeks.

Your transplanted hair will grow in  after three months and may continue to grow in for up to 18 months after the procedure.  The non transplanted hair which is your native hair will continue to grow after transplant.  As a post op protocol we recommend that our patients wait three weeks to visit a hair stylist.  The reason being, we want to make sure the scabs are gone and the donor is healed before subjecting it to a non safe environment.