I feed them when they're hungry.
I have two females who were eating every 4-5 days. I stepped them up to the next size of rat, and now they're going 6-7. I can pretty much tell by their attitude when I open the tub whether or not they're wanting fed or not.
I have another one who eats only every 10-12 days. I go by how she acts and feed her when she is interested.
I have one who eats about once a week. That is how often I feed her.
I only have four, so it's not a bother feeding on different days. But I have a feeling that once I get a bunch of them that everybody's going on some sort of a schedule.