I camped the Assassin/Supplier room last night. I can confirm that all 3 mobs in the room are on a 30 min timer. I even timed it with a stop watch =)
I was in that room for the better part of 4 hours. I had the supplier spawn 3 times and the Assassin spawn once. When I got there, another rogue was camping the room. He got the first mask and then left me (32 Rogue & 62 Druid) to the camp.
One thing I can tell you is to root/snare! if you don't and you can't kill the mob before they run off you WILL die.
I saw a lot of PH's but about 1 out of 3 cycles there was a supplier at least. Just wish I would have seen another assassin... oh well another night.
(on the upside i was camping foragers in TT before this and I was lucky enough to get the Iksar mask =)