I think that might depend on the reason for rejection.
Most people have an issue with their application being incomplete or not meeting certain criteria, in which case they figure it out early on in the process and the rejection happens early on.
However, I coukd see that people,who run into issues during their background check might get a late rejection, since that is only started later on during the process.
Check this thread and you should get a good overview on rejections:
The legendary Asivad Anac was rejected around 15 weeks after AOR (over a technicality related to his finances, and a few weeks later he received PPR).. I haven't heard of anyone being rejected post 15 weeks, although it's not impossible of course. It just gets less and less likely as more time passes.