In the Phoenix area, around mid-May the temperatures suddenly shoot up to around 103+ deg. F from the high 70's to mid 80's just a couple weeks before, and just get higher from there. It is a shock to the system and I find I need at least a couple weeks to acclimatize to the heat by taking some slower rides and then work back up to more intense rides in the heat. I learned that if I don't work up to it, heat exhaustion can creep up quickly within a half hour of an intense ride. It's not fun to hit a stop light and feel a sudden urge to both pass-out and puke.
Once that is through, and as long as I bring plenty of hydration and have some parks in the area with water fountains to re-fill bottles, I find I can ride for hours no problem in weather up to about 110 F. Anything higher than that is inviting trouble if you ask me. I do find myself losing substantially more salt in the summer as evidenced by all the white that accumulates on my helmet, so I usually switch to all electrolyte water during the summer months (aka 5 months out of the year here) -- something to watch out for as well.