The command works in 24 hour intervals. So 510 minutes + 15 seconds + 929 and 45 seconds = 1440 minutes = 24 hours
So your command is saying stay off for 510 minutes (8 1/2 hours) and then turn on for 15 seconds then stay off the rest of the day.
I thought I was using osc for my skimmer cleaner but I just looked and this is my code, old school style.
Fallback OFF
Set OFF
If Time 06:00 to 06:02 Then ON
If Time 12:00 to 12:02 Then ON
If Time 18:00 to 18:02 Then ON
If Time 00:00 to 00:02 Then ON
Lets take a look at the components of the OSC command;
OSC MMM:SS/MMM:SS/ MMM:SS Then ON
The first section MMM:SS is the initial time off command in minutes and seconds.
The second section MMM:SS is the time the pump will run in minutes and seconds.
The third section MMM:SS is the second time off again in minutes and seconds.