It's a lockdown timer. Once fight exceeds twenty minutes, you'll be locked in Worldstone chamber with Baal. Actually, you will be able to teleport to town one last time, but then you can't enter the portal back to Baal, it's effectively the same as save and exit.
Many of the lategame uberquests have lockdown timers as well, though they're usually much less than twenty minutes. It's done to limit "corpserunning" (playing suicidally just to wear boss off).
But honestly, your char is supposed to beat Normal Baal in much less than twenty minutes.