Similar to above, when I raced using NiCds back when dinosaurs still roamed the earth the measure was "comfortable" warm - easily gripped tight in the hand without feeling unpleasant. We used this for charging too.
I do the same test with my lipos now, if they are too hot to hold tight then something's not right somewhere - poor (high resistance) connection somewhere, poorly motor, poorly battery, poorly ESC or simply a lipo that doesn't have a high enough C rating for what you're asking from it - or way past it's best.
Never found an official figure but heat is the worst enemy of most electrical devices.
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