Here's the complete list of TF's and Trials in the game with levels and approximate kill-all times for reference (many of these can be shorter stealthed etc.):

TASK FORCES & TRIALS
Positron (Steel Canyon) - 3 minimum, lvl 10-16, 6 hours
Karsis (Hollows) - 8 minimum, lvl 12-15, 90 minutes
Synapse (Skyway) - 4 minimum, lvl 15-20, 5 hours
Sister Psyche (Indep. Port) - 5 minimum, lvl 20-25, 5 hours
Moonfire (Striga) - 6 minimum, lvl 23-28, 3 hours
Citadel (Talos) - 6 minimum, lvl 25-31, 5 hours
Ernesto Hess (Striga) - 6 minimum, lvl 25-30, ???
Katie Hannon (Croatoa) - 6 minimum, lvl 30-34, ???
Manticore (Brickstown) - 7 minimum, lvl 30-36, 5 hours
Numina (Founders) - 4 minimum, lvl 35-38, 5 hours
Imperious (Cimerora) - 6 minimum, lvl 35-50, ???
Mairenn MacGregor (Atlas Park) - 1 minimum, lvl 38-40
Woodsman (Eden) - 4 minimum, lvl 39-41, ???
Dr. Quarterfield (FBZ) - 8 minimum, lvl 40-44, 12 hours
Sara Moore (Cascade Archipelago) - 8 minimum, lvl 40-50, 7 hours
Justin Augustine (Chantry) - 8 minimum, lvl 44-50, 4 hours
Faathim the Kind (Chantry) - 8 minimum, lvl 44-50, 7 hours
Statesman (Indep. Port) - 8 minimum, lvl 45-50, ???
Lady Grey (RWZ) - 8 minimum, lvl 45-50, 4 hours

Twilight's Son (OB) - 1 minimum, lvl 25-34, ???
Mender Lazarus (OB) - 1 minimum, lvl 30-39, ???
Mender Silos (OB) - 1 minimum, lvl 46-50, ???

Respecs
Jane Hallaway/Save TV Reactor (Indep. Port) - 4 minimum, lvl 24-32, 3 hours
Capt. James Harlan/Save TV Reactor (Founders) - 4 minimum, lvl 34-43, 3 hours
Major Richard Flagg/Save TV Reactor (P.I.) - 4 minimum, lvl 44-50, 3 hours

(If you see anything here that needs updating, let me know!)

_________________

Global: @Sultry Siren

Dr. Khan TF(Founders Falls) - 4 Minimum, level 45+, ???

Kahn can be 45 mins to about 2 hours.

Statesman is 2 hours, 30 mins at the least on most teams.'

Imperious is 1 to 2 hours, sometimes under.

...Unless the ??? means it varies

The Marienne MacGreggor TF(Sewer Trials) Requires at least 4 people I believe.