After running some drives for a whole day, I noticed that power-on hours (SMART attribute 09h in some drives), has only increased 2 hours, which is really strange, so, I checked a manual for an Intel SSD, to find what does the SATA SMART attributes means exactly, and found this:
09h Power-On Hours Count
The raw value reports two values: the first 4 bytes report the cumulative number of power-on hours over the life of the device, the remaining bytes report the number of milliseconds since the last hour increment.
The On/Off status of the Device Initiated Power Management (DIPM) feature will affect the number of hours reported. If DIPM is turned On, the recorded value for power-on hours does not include the time that the device is in a “slumber” state. If DIPM is turned Off, the recorded value for power-on hours should match the clock time, as all three device states are counted: active, idle and slumber.