I don't know what medicament you used, but with average pill you should perform worming twice: first time, and then 14 days later (it is connected with life cycle of some worms).
Then, if you rat goes out for walks, repera worming every 3 months. If it stays at home - every 6 months (it is necessary, because you may bring worm eggs eg. on you shoes etc., or some parasites, ie. tapeworm, may be brought by fleas).

What you are describing- large appetite, weight loss- could be a sign of hyperthyroidism not worms. If it is, there is a holistic way of dealing with it rather than the more toxic conventional approach.