I've sprayed my peach trees with home made Burgundy mix (a variant on
Bordeaux) and was wondering how long copper sprays have to be on the plant
to be effective?

We got a very light shower within 24 hours of application but I can
still see a light blue tinge around the bud edges and it doesn't look if
much, if any, got washed off.

Given this situation I began wondering about effectiveness and not one of my
gardening books, of which I have oodles, says anything useful about how long
copper sprays need to remain on the plant to be effective as a fungicide.
Anyone got any idea?