That's a good question, You know I never checked it, (which is stupid on my part) but I can tell you that I have always ran both the Briggs and the generac 100% and only stopping when it ran out of fuel for days until the gas climbed up over $1.50, then we started becoming more fuel management due to fuel cost and not duty cycle. I know tons of people that run them 24/7 until power is restored and I think the manufactures know this. Without checking I would think it's 100% duty cycle, the manufacturer knows there's a lot of completion out there and they are always striving to make the most selling generator on the market. I just know by trial an error that neither generator has let us down yet.

Update: On both sites I believe it is 100% duty cycle.

Briggs site
http://www.briggsandstratton.com/engine_power/details.aspx?pid=146


And my favorite,the mother of all generators, the generac

http://www.generac.com/Products/Portable/UltraSourceSeries/UltraSourceSeries.aspx

If I could do it all over again, I would buy the Guardian 17.5 KW