The Tapeworm Diet website has an awesome FAQ:
How much weight can I expect to lose?
Weight loss varies from individual to individual, but for a single tapeworm you would expect between one and two pounds a week as the worm matures. I know of instances where individuals have super-infected themselves and lost weight at a faster rate, but have also suffered a much higher incidence of side-effects. It is important to be realistic about weight loss and a rate of a pound to a pound and a half a week is ideal.
Will I keep the weight off when I kill my worm?
That all depends on you. I have noticed a significant rebound effect especially when hosting multiple worms. When you infect yourself you will get hungrier and eat more. If you reinforce poor eating habits during your infected period you can expect the weight to return when your worm is gone.Telling people you're "hosting multiple worms" is without a doubt the best conversation starter I've ever come across.