Nitrogen problem

hey if anyone could please tell me in how long i should see a difference in my yellowish leaves once i changed food and added more nitrogen

depends on how bad they were..
if they werent too bad, it could be a few hrs, if the yellowing was bad they may never recover.
Just add a very diluted dose of nutes, and see if they pick up...if not, try a small diluted
amount of nitrogen.Dont apply it to the leaves,pour it into the soil..
Dont use the recommended dosage cuz it could fuck em up big time..
its easy to lockout the other nutes by being heavy hande with N.
Patience is required here, your plant could be stressed therefore not taking up the fresh nutes properly yet..