What if you pass the selections, but have to cancel in the end (because of illness, job, personal reasons,...). How much time do you have to let the organisation know and will the place be given to another person so it wouldn't get wasted?

We know that around 25% will dropout. Some of silly reasons like "i didnt plan this thru" and some of serious reason. Shit happens, we know that. If it will be fast enough and there will be free places, then yes, we accept more ppl.
And how much time? As fast as you know you cant come.