I tried to check the service menu of my 508XG, naka 100+ hours na din ako ;-). Below is the procedure from AVSforum (I just edited to match w/ 508XG):
WARNING!, do not change any settings, just view the settings
Start with the plasma in Standby
Make sure you point the remote directly at the IR sensor in the tv
Press INFO on the remote (just press, do not hold)
Wait 4 to 5 seconds
Press LEFT, UP, LEFT, RIGHT then POWER
If done correctly, the plasma turns on and the INFORMATION page is displayed
Then use the down arrow to page through to page 5
PANEL = the time the set has been on (format is 100H45M; 100 hours 45 min.)
MTB = the time the set has been plugged in
When you finish press the HOME MENU button