I'm curious to see what answers you get from the more experienced folks here.
I will say this though, and understand that it shouldn't be taken as the final word by any stretch, just my opinion.
I typically don't like the concept of quoting duty cycles. It really comes down to how much of a risk you are willing to take. When the host (near the heat sink) starts getting warm, it's time to turn it off and let it cool awhile. The larger the mass of the heat sink / host the longer you can go before the host starts getting warm.