The creatures will continue to spawn as long as the area remains accessible. The area closes and players are kicked out approximately 10 minutes after the pale count has been killed. SO there is time to still make the bestiary entries if you weren't one of the first people to show up.
EDIT:
Worth to mention, that such a huge amount of monsters we will face only at the begginging of the raid and it spawns only once. After we kill all of them respawn still works, but monsters appear slowly and there is not many of the new ones.