The cactus is ready when it reaches the size that you want. Depending on the potency(highly variable) a one foot cactus could give from no effects to moderate effects. Just decide how much cactus you want to eat, and the intensity of the trip you want, and wait til the cactus gets to that particular size.

And you can't boil water? Or you you just don't have access to the kitchen without someone else being there? Can you go camping? Make a campfire and cook that shit up and trip in the woods. That would be badass.