Anytime !!! That is the nice part about nitrous. As long as you do not exceed what your car can handle it can be added to a stock car or a highly modded car !!..
I had nothing done to my car except an intake and a 50 shot of nitrous and was beating medium modded cars and svt's on a regular basis at the drag strip...
Speed is only a matter of money; how fast do you want to go?