All microwave ovens fire differently. The firing time varies from one microwave to the next, and a MagicFuse kiln that is still warm from a previous firing takes less time to fire than a cold kiln. A fluctuation in voltage can also change the firing time.
Five minutes is the recommended minimum firing time. Use a reduced power setting on your microwave if it fires in less time than 5 minutes. This will lengthen the firing time to give you finer control. See the microwave manual for instructions on reduced heat settings.
The easiest way to fire the kiln is to fire a glass piece in several stages, visually checking the glass at each stage. Each time you visually check the glass, fire 30 - 60 seconds longer until the glass has fused to your satisfaction. Start out underfiring a test piece, and gradually add more firing time.
The glass begins to fuse during the last 30 seconds of firing. This is why precise fusing is much easier with visual inspection of the glass.
Keep a detailed firing logbook to record the firing times for each type of project. Write down the following:
Type of microwave oven
Brand, color, and size of glass
Pre-heating time, if any, and pre-heat microwave power setting
Power setting during firing
Whether the kiln was still warm from the previous firing
Firing results and what you learned from the firing