Please mind that in order to count your gigs towards quest goal, you have to have a proper gig account connected (for example, to count your gigs towards Uber quest, you need Uber account connected to Appjobs Work, etc.).
There is no limit in case of how much quests you can join and participate in at a time.
All quests have set time limit. The time finishes at the exact set hour, regardless of when you join. For example, if you join a quest at 2 PM and it ends at 11:59 PM the same day, you'll have only 10 hour to reach the goal.