Any session will remain valid for 2 hours after the last request to QuickBlox. To check a session's expiration date use this next snippet of code:
NSDate *sessionExpiratioDate = [QBBaseModule sharedModule].tokenExpirationDate;
Check this guide http://quickblox.com/developers/IOS#A_couple_of_words_about_Authentication_and_Authorization
This feature is available since 1.8 iOS SDK