What was the average duration of a Me-262 combat mission during WWII?
This depended strongly on the flight altitude. Staying at low level for the full mission meant that the Me-262 would run out of fuel within 40 to 50 minutes.

Climbing above 6 km (20,000 ft) would extend that time to 90 minutes. Time to climb to 9 km was 13.2 minutes, so that would leave a time for engaging the enemy of 60 minutes. Considering that typical armament were 24 unguided rockets and 4 cannons with 80 or 100 rounds means that the 262 would run out of ammo in much less time.

One pilot told me that a typical training mission would start in Munich, continue to a radio station near Berlin and back to Munich, all within 60 minutes.