Hard to say. Each heli setup is different. My 550X I'd fly in just about any heat I'm likely to encounter here in NY. Same on the Goblin 500 and anything smaller. My TRex 700 gets very hot though, both motor and ESC. I might fly it a flight in 90+ heat but i'd give it a good long cool down time and be particularly careful not to overfly my timer so the batteries don't get too hot.
If you have an optical temperature gauge I'd use it on the motor and ESC after a flight during a good day or perhaps in the evening if the temp comes down, to establish some baselines. Then try a flight in the heat and see where things end up. In general, if the motor is getting up to 150+ I'd be starting to worry. ESC 175+.
Another option, lay back on your flights on the hot days. Don't be pulling endless streams of hard collective tictocs. Mix things up more with some slowish big air passes. Give things some clean air and low load to cool in flight. And give plenty of cool down time between flights. This is particularly important for the motor. The bearing oil will break down with higher heat. If you fly hard flights back to back you're going to break it down even more quickly. Let it cool between flights and make sure to keep up on bearing oiling maintenance if you're consistently flying hot days.
