Possible Nitrogen - Recovery time+amount?

I've had what was diagnosed as a nitrogen deficiency in about half of my crop.(4 weeks into flower)
I'm 5 days into treating them with a nitrogen based fertilizer as advised and was just wondering what sort of recovery i should be seeing if the diagnosis was correct? and how long i should treat for?
The plant that was effected the most seems to be showing no color change in the affected leaves, however new growth seems to be a healthy green.
Will the old leaves recover their color or is it just new growth as i'm seeing that will be ok? How much will this affect the plant/yield for the rest of the cycle?
thanks

here's some pics .
1/a leaf from a normal plant next to a leaf from the worst affected plant.

2/ the affected plant next to normal plants

In flower, we all cut way back on nitrogen. Looks more like underfertilizing, but it would be best to fill-out the CanCom troubleshooting form before making any drastic changes based on a guess. (the link is in my signature)

I'm curious...who diagnosed it as a nitrogen deficiency?

it was the guy at the hydroponics shop that diagnosed.