The summoning spells now last 24 hours outside combat, but they will only last up to the former duration when actively in combat. If they stop being in combat, that duration pauses and will only resume once they are in combat again.
Example:
A summoned dire wolf has a duration of 15 rounds. It fights for 8 rounds, leaves combat for 2 hours then enters combat again - at which point it'll stay around for 7 rounds (except if leaving combat again.) However, no matter if it fights or not, it will only ever stick around for 24 hours.