What is the best time for pruning camelia and how much can I cut back

As soon as its finished flowering.In the past I have pruned camlia quite hard and they have come back ok.You may not get many flowers next year though.Give it a feed of sequestered iron and talk nicely to it!

How big is it? Why do you want to cut it back? Agree about the feed. they like acid soil so its probably not been treated right. I would concentrate on getting some new green leaves. Best of luck!