I can't really answer that question. But let me make a comment. None of the dongles I I ever had got hot when powered directly from the USB on the computer. But, two weeks ago I bought a powered USB hub to insert my RTL sticks into so I had one handy place for them all. When i have power connected to that USB hub, the sticks run super hot. I'd venture to say their lifetime is going to be shortened if I continue to use the USB hub in a powered state.
When I unplug the USB hub power, I can still run two dongles on the hub [I didn't try more] and there is no longer a heat issue.
Do you happen to be using a powered USB hub? The last two RTLs I bought were the Nooelec Nanos. And when plugged into the USB hub with the USB hub powered by the adaptor, the antenna jack gets so hot to the touch that it can actually sear the skin. Fire hazard for me considering how much useless stuff I have packed into the area where the USB hub is.
Just using the power from the USB port on the computer, I've never had / noticed a heat issue on a cheap RTL stick or an FCDPP.