Duty cycle....

It is based off of an hourly cycle, correct? Duty cycle is 15 minutes that means you can use this device for 15 straight minutes followed by a 45 minute rest, correct? Have a grinder for my chains on my chainsaws. During cooler months, when the garage is 30* or so the thing will run non-stop. Hotter months, if it runs for 7-8 minutes before shutting down that is pushing it. Company I got it from is telling me this is due to its duty cycle. I get that but what I don't get is I have left it running for 4 straight hours in cooler weather....I forgot to turn it off...without it shutting down.....