As Deb said, don't try offering food everyday. Since you are on a five day schedule offering every five days would be fine. Hardly any of mine will take f/t of thongs, it's only the really good feeders that will do that for me. What I do for f/t is put it in at night and leave it in all night. The part that sucks is that if they don't eat it you have to throw it. You could always put it in late and get up after a few hours to check on them, and put the rat back in the freezer. My rule of thumb is to only re-freeze once. I've wasted a lot of f/t rats, it's shameful, but it's life I suppose

As for how long to hold out. If they have some weight to them I say a month easy, but make sure you are only offering once a week so they don't get stressed out.