Return of Reckoning

How long is it from killing a lair boss and to when it respawns? This is can get pretty annoying when you don't know when it is going to respawn and your just sitting there for 15 minutes doing nothing. Also Is there away to make the wait time shorter during events like wild hunt when more people would be doing them?

The lair bosses in tier 1 (that drop tier 1 items) currently have a 15 minute respawn timer. Everything else has a ~2 hour respawn timer.