Data is for clean (no ext stores) MiG-21 bis:
Climb with full reheat till 600km/h, mil thrust afterwards.
alt (m) time (min:sec) fuel used (liters)
1000 1:35 245
3000 2:05 300
7000 4:10 425
10000 7:00 540
Cruise consumption (liters/hr)
alt (m) max range cruise max endurance
500 2570 1900
5000 1870 1560
10000 1510 1430
However, Mig21 is very complicated (in civilian terms, that is) airplane and despite its massive fuel consumption, fuel would be one of the least expenses for its operator.