it depends on the severity of the deficiency. I've had nitrogen deficient plants before (lower fan leaves yellowing) and it started looking better in a few days after feeding if the leaves were somewhat yellow -- as in at least still 50% green. Leaves that are completely yellow most likely won't return to being green.
try feeding your plant every other watering and see if it starts looking better in 3-5 days. without seeing a pic of the plant it's hard to say what's up with it, but I'd assume it used up most of the nitrogen in the soil by now since you said you're over 2 months into the grow without any nutes.