As DrusillaTheEnchanter mentioned, there are lockout timers for each type of trash mob in your expedition window, which is beneficial if you're doing Hunter achievement as a group, because restarting the mission will clear that type of trash mob from the zone. There are timers for 17 different types of trash mobs plus the 10 different Hunter mobs. If you got all 27 lockouts and started a new mission, the only mobs left in the zone would be the 5 raid mobs. Lockout Time is 4 1/2 days each.
Once you lock yourself out of enough trash mobs, you'll get this message every 5 minutes: "There are an insufficient number of minions in this plane to weaken the planar gods." which is only a problem if you're raiding.
These are the 17 trash mob types with lockout timers in Plane of Fear Revisited (TBM):
Amygdalan Knights
Boogeymen
Fetid Fiends
Frightfingers
Glare Lords
Gorgons
Nightmares
Phantasms
Phoboplasm
Samhain
Scarelings
Shiverbacks
Spinechiller Spiders
Tentacle Tormentors
Thought Bleeders
Turmoil Toads
Worry Wraiths