I did some number crunching today and I found that in CORE v.0.84, the first German jetfighters and rocketfighters can be ready by summer 1943. The Me- 262 (cool plane) could be ready by summer 1944. Quite historically as far as I know.
870 days (2,5 years) of rocketry research,
3090 days (8,5 years) of fighter research,
2160 days (6 years) of air doctrine research,
all going parallel until 1944 and resulting in the Me-262 model.
I also calculated It would cost an average of 36 IC in all those 8 years for fighter and rocketry techs and doctrines. That includes a rough estimate of the {8,...,25} IC usage for the application techs in all three branches, the actual planes you need as you go along etc. that gives appr.16 IC over 8 years. Of course I forgot one variable... It does not include the arty-techs you need to get the 30mm aerial gun. The total jetfighter budged would be somewhat larger since it includes both air, rocketry, artillery, and air doctrines.