how long does discernment last is it up to me or a spiritual advisor?
Well each calling is unique, but when one enters the seminary is decided by the Vocations Director.
Discerning doesn’t stop at any one point. It’s a common misconception to think it stops some time.
Anyway, concerning entrance into a seminary; one would need the approval of the Vocations Director or the Bishop. If you haven’t already, contact your local Vocations Director and chat with him. He would be able to fill you in much better than I.