Once a floor boss is killed, does that level become bossless or does another boss appear with time? The dungeon walls create monsters constantly, but how about the floor bosses? Once a floor boss is killed, does that level become bossless or does another boss appear with time?
When a boss is defeated, it will appear again after its recovery time. 

That means while the boss hasn't recovered yet then that level is technically bossless.

You can see this in season 1 episode 10 of Danmachi


   Bell and his teammates are having a hard time because of pass parade then Lili suggested to go in floor 18th since it is a safe zone. Welf is concerned about the boss in floor 17th but Lili said that they can make it since the boss is still "recovering"