It's just like CRI, they say x amount of hours, but if you turn it in the next morning they still take it.
For Marketforce it's anywhere from 1-24 hours depending on the assignment and if you are in blue or purple portal. Mystery shops are typically 12 hours. But they don't bind you to it or start bugging about when are you going to get it done right away. If you were to forget about a shop or not have time to do it because something came up, you can usually reschedule it as late as the morning after you had planned for. For time sensitive assignments, I would not recommend doing this though. Just don't make a habit of it because they won't tell you about any penalties they have against you. If they have issues with you they will ban you from doing assignments (this could be all or just certain types) for a period of time or indefinitely. It could be a problem with one shop or multiples that causes this as well.
For instance, I couldn't complete a trailer check at the last minute due to lack of child care. They felt it was reasonable to remove me from self assign for those assignments for about two weeks and unassign 5 trailer checks I had lined up. I had been doing them regularly for two months at that point with no issues. These assignments say to submit them within 1 hour of completion and always have to be Friday morning. I can't really blame them for penalizing me, trailer checks are highly time sensitive. But it was a bit excessive.