If your legs start blistering, it's too hot.
The Bandit engines will throw off a lot of heat when it's hot outside, but I've never heard of anyone damaging an engine.
Keep good oil in it, avoid letting it idle for extended times in hot weather, or getting stuck in real slow, stop and go traffic.....and use your common sense. If you get stuck in stop and go traffic, and your legs are cooking......try to find another route that will let you move at least a slow pace....or park the bike and shut it off for a while.