The question of how long you can run XXXXXInsertCompoundXXXXX always has the same answer.
That answer being, until you die.
Now a more accurate question is whether running XXXXXInsertCompoundXXXXX will hasten the condition and point to which you can't run it anymore. That point being, of course, when you die.
That being said and hoping to give a new line of thought for long term planning (when I say long term planning, I mean like in years, not just PCT mode like some newbies focus on).
I would say with proviron (or anything really), run as long as you need it. Not as long as you want it.
50mg daily isn't going to change your body much in the mirror or gym. But if you need it post cycle for recovery, use it. If you need it on cycle for dick problems, use it. But if you are 4 months off cycle and can't get a hard on, don't use it. Because there are other reasons why you may be shut down and you need to find out why instead of just putting a Band-Aid type action over it.