It really depends on a lot of factors, namely the age of the plant, type of root tabs, and the intensity of lighting.
In the tread I started on some diy root tabs I made (I'll post link when I get to a pc) I posted a picture time line of my amazon swords growth. This was a plant that hadn't been moved in 6 months so it had a very well established root structure so responded very well to the root tabs. A newer plant wouldn't respond as quickly.