Do I have to boil my Wonder Warmer RIGHT after I use it?

I just ordered a Wonder Warmer hand warmer and I would like to take it to school to use in the cold mornings, but I just found out that you have to put it in boiling water after you use it to melt all of the hard crystals, so would I have to go home as soon as the heat fades, and throw it in a pot of boiling water for ten minutes? Or can I wait til I get home at 3 to throw it in (and have it hard, in my bag all day)??? I found out about these things from a girl at school who had one and used it at lunch, but I'm unsure...