There’s not a strict…afterlife, really, but there is a place where souls can go between incarnations. It’s closed off to any being not purely a soul, which means it’s inaccessible to Alcor. If a soul decided to stay in that other realm for any amount of time, he’d be unable to do anything about it. If Mizar did it, he would probably spiral a bit–even if he shouldn’t depend so much on her existence, he does it anyways.
In terms of length of stay in that other place, it varies. Souls typically don’t stay forever; some souls bounce back for a short while and then get out to go live life again, some souls take a bit longer, others take ages, and then maybe there are souls that stick around to only occasionally live.