Using the Agent tweaks mod to see Fatigue status I found these results:
Thomas Grady
~29% Fatigue
Turn in one 15m mission
~45% Fatigue
Turn in one 15m mission
Incapacitated with 9hr 50min cooldown.
That seems rather extreme and unpredictable.
Edit:
Faction Recruit
Fatigue: 45%
1 hr mission turn in - Success
Incapacitated for 8 hr, 38 min, 30 sec
Prior to this mission turn in, I did two 15 min mission success turn-ins and they upped fatigue by around 10-12% each time.