Its all about curiosity and non-commitment. Everyone wants to have first crack at seeing it, knowing they can bail out immediately if they don't like it.
How long it will take is an unknown. The longer one plays, the more chances of doing something that uncovers a bug. Bugs are found by doing something unexpected, so how can anyone predict how long that would take?
"thoroughly" means playing LOTS and trying as many variations of the same actions as you can possibly think of. Moving everywhere and anywhere and at different times relative to other objects on the screen. Mashing the keys and attempting strange key combinations, etc. It can go to infinity, so yea... prepare for hours and hours of testing. That's why one wants lots of testers; because each has limited time.