Dear Keith,

Thanks for the question.  

Typically, penances are rather short and easily completable, so I'm guessing this "prescription" was meant as an ongoing way to spiritual health.  It is almost like a spiritual director giving you guidance on how to better perfect your spiritual life.  I would say that doing this imperfectly does not invalidate the Confession because penance is not open-ended, as this is.

If, in Confession, a priest said, "Pray the Rosary every day," I would do my best to make that part of my routine but missing it here or there, doesn't negate the sacrament.  It is the same principle.

Priests mostly give very short prayers that can be completed in a few minutes.  I think this priest had a different intention in his instruction.

Peace,

Bob Kirby