There is no restriction on the casting time of spells that you can imbue into an item with Prime 3. For permanently active effects this is not a problem, but in the case of conditional powers I am unsure about the time required to activate them.
A trigger should be an appropriate instant action [..] When the trigger is activated, the imbued item’s spellcasting dice pool is rolled. (MtA 84, emphasis mine)
Does imbuing a spell into an item enable you to effectively turn a spell requiring extended casting into an instant one? Is the pool rolled only once, regardless of the original casting time?