when do you prune a quince?

Hi Amphibian_heath, welcome to GOY. You should prune after harvest but just enough to keep the tree from being too dense and get rid of dead wood. The fruit is borne on the younger wood so if you prune too much you won't get much of a crop the next year. If the tree is really old and overgrown you can cut it back fairly drastically and it will rejuvenate.

John.

Now, the problem here is what you mean by a Quince tree. If you mean a Chaenomeles japonica then you can prune it as suggested, this forms a bush rather than a tree. If you mean Cydonia oblonga, the true Quince either Meeches Prolific or Vranja say, then they are grown as standard trees and need different pruning.
On trees you as said remove and dead or crossing branches to open out the middle of the tree, otherwise just cut back the new growth to a bud to encourage lots of new growth with flower buds on it.