Probably not that long. Rat metabolisms are really fast so they gain and lose weight quickly. As for their coat, I think I noticed the change in my girls after about 3 weeks.
I don't know if this was mentioned previously in the thread (and I'm too sleepy to go back to look) but if you're getting a big bag you may want to freeze some of the food to keep it fresh since you are only feeding 4 and they wont go through it too fast.
You said you have 3 babies, and depending on how young they are you may want to give them a small but daily treat with higher protein as a supplement to make up for the low of 14% in oxbow. But only if they're still growing. Then again, some people don't feel it completely necessary to do that in young ones, and find 14% to be just fine.