Bay Laurel Pruning

I have a nice bay leaf bush. Two stems have shot up about three feet above the rest of the bush. When would be the best time to prune the bay bush ?

As long as you leave a sizeable bush with a good number of leaves, you can prune it any time. If you prune in the fall, the bush will stay in its desired shape for the whole winter. If you do it in the spring, the next growth spurt will regrow at least some of the height with about twice the number of branches!
If the bush has few leaves, then shorten the branches only a bit. It will encourage branching without seriously weakening the plant. Doing it in the spring would then also be a better option, because the bush won't have to photosynthesise for such a long time before new growth adds new leaves.