Actions on the battlefield in Attila cost stamina, which increases fatigue, which reduces speed, melee attack and reload. Using running, which has a cost of 9 , we can reverse engineer how fatigue is calculated. It seems to be cost x amount of seconds doing activity x 10.

The stages of fatigue are:
Fresh - 0 no penalties
Active- 1000 -5% to speed - reached after about 11 seconds of running
Winded- 3000 -15% to speed, reload, melee attack - - reached after about 33 seconds of running
Tired- 6000 - 20% to speed, reload, melee attack - reached after about 66 seconds of running
Very Tired- 9000 -30% to speed, reload, melee attack - reached after about 100 seconds of running
Exhausted- 15000 -35% to speed, reload, melee attack - reached after about 165 seconds of running.

How quickly units regain stamina coming right up.