15 minutes before landing. This would typically put the aircraft at or above FL100. At that time, there still should be some pressure differential putting a load on the exit.
It has been a while since I have last flown the Fokker. But I seem to remember that the overwing exits move in a little bit when one pulls the lever - so looking at the picture, I am distinctly under the impression that this has not happened on its own. On top of this, in several years on this type (both F70 and F100), the few times I have had (or even heard of) an overwing exit open without good cause have all been passenger related. Reasons ranged from curiosity that killed no cat, but cancelled the flight to a passenger that felt the need for some fresh air just after boarding and only saw the problem with opening the windows on an airliner after a stern lecture.