I generally am fine on hot days when I ride after work, starting around 6:30 pm. On very hot days I will wear my cool wings and wet them once or twice during the ride. My usual post-work route is mostly on quiet streets that are noticeably less hot than the bike path next to the highway where I start/finish.

For daytime rides, I'm usually okay if I avoid riding between 1:00 and 3:00 -- the position of the sun in the sky is more important for me than the temperature. I did learn the hard way that even 10:00 am can be too hot for riding after several days of 100 degree highs.