I have a 3 (soon to be 4!) tiered espalier apple. I understand about summer pruning but what I don't know is what to do when each arm of the tree reaches its final desired length. How do you stop them from growing anymore?
They will keep on growing, that's what nature intended! :)
However, you will need to continue a regime of pruning. If you prune in winter, this will encourage the plant to grow even more vigorously. Best thing is to prune the ends of the branches you want to control in say, end July. They may put on a little growth which may be affected by frosts later in the year, but this won't matter too much if you want to restrict the length of the branch.
As JennyM says summer prune only and cut the tips to an inch of last years growth or the previous bud if they are getting too long!
;D