Depends on the keys. for lower keys like 20+ and under even if you don't time it most people will stay to finish it to fill their vault (unless you have like 20+ wipes). for keys above 20+ is pretty common to leave since at that point people are looking to increase their IO not play for vault.So far I haven't had a party that disband yet. Heck once even the tank went afk for a a few min coz he had to change the diapers for his baby. i teleport out change to change spec and tank till he got back. Sure the run was like 2 to 3min over but no one left since we just want to fill our vault.
I been pretty casual myself and join casual guild where most of us are adults have families and some have kids. Sure we just started pushing heroic raid but we all have fun and that's the important part.