Some auto tranny fluids will start to flash in the low-to-mid 300°F range, typically most by 350°F. Synthetics usually are safe above 400°F. ATF datasheets spec viscosity at 100°C. I guess the warning light really is a warning light, so if it comes on you are taking a ride in the danger zone and like Steve says, flush and fill would be wise because you might be cooking the lube.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler