Duty cycle and derating curve

Team,

In our DMD datasheets we are showing a derating curve for different duty cycles (micromirror on/off time). But it's not mentioned during which time period this has to happen. Is it per second, per hours, per day? So if I have a mirror continuously on for 4h and then off for 4h, this is sill 50% duty cycle. But does this fulfill our requirements?

Thanks & best regards,
  Robert