Just use Breguet range equation to do some calculations for range estimates. Feed in some feasible L/D, SFC or fuel fraction (e.g. compare with other aircraft published data) then see what you get out.
Well it's actually bit more complex than that.. Used as it is. The breguet range would give "ferry range" which is... the "1 way" range. More iterations are needed. Like fuel utilizations, calculations of SFC during combat and returning to base, the aircraft's L/D's.
But anyway did that in excel with following assumptions :
Empty weight : 17000 Kg
Fuel weight of : 11000 Kg
L/Dmax of : 11 (looks reasonable, as far as i see typical fighters are about 10-13)
Cruise L/D is assumed to be 93.4% of the L/D max
Payload of 4000 Kg (weapons, pilot, etc)
Mission altitude of 12000-15000 m
Also Fuel utilizations need to be defined too. The radius or combat range is then effectively the cruise fuel.
The plane will "supercruise" throughout the mission at Mach 1.2. and doing afterburner during the combat as it would need some maneuvers. Then RTB, the fraction for RTB can be less than cruise phase one as the plane would drop payload. so it's lighter.
So what i got was that the combat range for my hypothetical J-20 there is 1384 Km. With enough fuel for 10 minutes of combat or dogfight with afterburner, and enough fuel for RTB with 6 minutes allowance for loiter awaiting landing.