How long?

I bought four small females two weeks ago and I'm wondering how long it will take for them to grow into breeding size

I also wonder how a rat would do on dog food... Even though it contains 21% protein in it.... Is that bad for a rat?

Roughly 2 months and you just need to know that the first litter may only be 6-8, but then they will go up from there to about 12-20.

I would wait at least 8 weeks. Feed them freely to make sure that they grow as fast as they can.

Some dog food is okay for rats and some isn't. Hard to say without seeing the label.

Letting females become pregnant too early will definitely stun their growth. Id say keep them separate until 3-4 months.

Protein and Fat:
Pups (under 3 months) and Nursing Mother (18% Protein & 7% fat)
Young Males and Adult Females (16% Protein and 5% Fat)
Adult Males (14% Protein and 4% Fat)

I avoid dog foods becuase rats do not need that much meat content and the fat & protein is far too high. Too much protein can hinder lactation, dry out skin and cause protein scabs. Not nice at all. If your rats get too fat you won't have any mating and the males are more likely to have hind leg paralysis.