Usually at the first signs of it occuring, or when the foliage starst into growth if you are prone to getting it. Clear up any remaining dropped leaves if there are any about.

Spray with bordeaux mixture in spring before any blossoms or leaf break. Then spray again a little later once the fruit has set.
Peach trees are very prone to this disease and the only way to grow them successfully without spraying is under cover, even a winter cover which can be rolled down over the tree from a wall.
We have one tree that never gets it which is beside the house and partly under a sloping roof. All the ones in the open get it very badly unless we spray.
I did read about a peach leaf curl resistant variety last year but can't remember what it was called,or if it really was 'resistant.'

thank you all very much