When a spell takes multiple turns to cast, the player stays locked (can't do anything with him while casting). Also, the spell will show up in the turn list (showing you where your character spent his turn still casting). In this case, you cast a summon, then the next turn you get to move the same character again. The summon never fires off. If you cast the summon again, it will work again.
I've only tested this with summoning an earth elemental. So, results may vary.