If you have a cooler, and are growing hair on one...the effects are really pretty subtle. You are putting on muscle and hair at the same time so it isn't as noticeable. You will notice in a couple or three weeks.
Optiflex can be fed for UP TO 45 days I believe. Zilmax was the 20 day deal.
Start them plenty early and watch like a hawk. Make SURE they are getting exercise and not just laying down and going to sleep. If you notice them getting tight in their movement and shortening their extension of stride, just stop feeding it.