If you are talking about re-heating plastic after if hardens, this is what works for me.
Like Del said, I cut the plastic into small chunks. I found that if I have 1/2-3/4 cup I can put my microwave on power #8 for 2:22 (usually) and my plastic will be ready to pour with very little bubbles. If I get a lot of bubbles then I'm heating it too fast or too high. Like Prochallenger said, there are just too many variables to really get it down pat. The type of plastic being used, even the color. I find that recipes with more drops of color take longer to heat, etc. Lighter colrs are not as forgiving. Try not to reheat too fast and work your way up to the pourable state. If you get a lot of bubbles, stir for a bit and set it aside. The bubbles will disappear. Some people skim the bubbles off or even zap it with a flame. I set it aside and wait. With my luck, if I used the flame, I'd probably burn down the garage! The more you practice, the more you'll figure out what you can do with what you're working with. If you have the time, spend it searching old threads. A few hours reading the old threads will save you months trial and error and a ton of wasted money. Sorry for babbling, but I just drank some Cuban Coffee!