You can cut these back to any point on the stem at any time and they will eventually sprout and come back.

Albeit not very quickly.

You can prune it any time of year, you will just sacrifice next year's blooms doing it this late in summer.

I had to do the same thing when we moved into this house in the Fall. Some genius planted a camellia about eight inches from the foundation and it was HUGE!

It has come back wonderfully -- cut from its original 7 ft to about a foot. Now, four years later it is about three feet and nicely bushy. Has bloomed every year except for that first spring.

Camellias can be pruned any time of year, so long as you don't care about getting maximum bloom the next spring.

One caution about pruning shrubs in this heat, you will expose to the hot sun parts of the plant that have been shaded by the branches you're pruning off, and that can cause significant shock and stress. Wait for the heat to subside, and give the plant a good soaking when you do prune it.

Do what reg pnw7 said--wait until it's a little cooler. We did a massive pruning (in anticipation of window installation) last September and October, and everything did great.