For my GE micro, power level cycling is as follows:

HI/Full - Magnetron on continuously
90% - On 27sec, off 3sec
80% - On 24sec, off 6sec. etc etc.
50%/Medium - On 15sec, off 15sec
20%/Lowish - On 6sec, off 24sec

And so on.

I'm sure the designers took everything into
account when providing for all those power
levels. I almost never use Full, except for
when boiling water.