These enchantments generally last a random amount of time, much like, say, Deodorize.
Doesn't really matter, however, as there's a better option available: The Pilgrim's Wand gives +2 MP Recovery while Healing without the use of charges. Plus, since it's Rare/Ex and drops off of an uncamped, easy-to-kill NM, anyone can get it without spending a bundle. It may not offer the +2 Int/Mnd, but there are better options for that as well. Just create rest and stand macros to swap between two wands and you're golden.