I'd wager on slowly released. TES strikes me more of a universe where souls can never be truly destroyed, just recycled and reused.
The situation with Umaril seemed to be an exception.
I wonder if mortal souls can be destroyed or not. If the soul is slowly released when recharging, then I imagine each use of its charge tears off a piece and releases it. Perhaps that weakens a soul, but it mends.
If an item is enchanted, it would seem the soul is bound to the item.