TRT is usually a life long treatment.

There are some newer agents used (or about to be used) that can really jump start the HPTA and it's possible to start it up again with the available PCT drugs in many cases,
but your doctor should have informed you of the real possibility that if you go ahead with this, and stay on for any length of time,
There are chances you won't be able to quit using exogenous testosterone;
meaning you'll have to rely on injections for the rest of your life.