Melting the chocolate depends on the size of your chocolate pieces and the power of your microwave. That's why I recommend just heating for 15 seconds at a time and then removing chocolate and stirring. When you stir and there a few lumps remaining, just keep stirring and the warm chocolate will melt the rest of the lumps. As for hardening, this partly depends on how warm/cool your kitchen is. But it should just take a few minutes for the chocolate to set.