In addition to the overall time limit for finishing a dungeon (which is usually quite generous) there does seem to be another timer that kicks in after the last boss is down - I think this what the OP is talking about.

I know that once when I did Sastasha (NM) and we stood around talking a bit afterwards I was surprised when we got a message that there were only 5 minutes left.
The run had been a bit longer than normal since we did a complete clear of the whole dungeon, but nowhere near the 90 minutes that is the timelimit for the dungeon.