Only Jagex knows as nobody ever has any such data outside Jagex.
My guess is pretty high, given we need to pay either real money or bonds to play FSW. Of course, when players have to pay there have the highest tendency to get the best bang of the buck in continuing playing.
The other thing is the valuable rewards we can carry into the main RS3 game. If we stop playing, we simply can't realize that gain. It just doesn't make much sense.