Bay trees are grown in all manner of situations. As a large shrub - given enough room - they will not need basic pruning. As an ornamental - topiary - Bay Leaf Tree, they will need regular pruning to keep them in a tidy shape.
Winter damage is often a problem of bay trees, with browning of much of the foliage - also wind scorch damage. Both problems can be addressed by pruning as required to remove the unsightly branches. they soon grow back.
For topiary specimens of Bay Leaf Trees, it will be necessary to prune then twice in the summer months. April and then again in August are good times. If you prune your bay any later than August, it will be too late for the plant to re-grow new growth before dormancy sets in, so a final cut in August, will leave you with nice fresh foliage for the autumn and winter months.
The first pruning session in April, can be as severe as you wish - if you want to head back untidy growth. This is also the time to start with a bay if you are going to prune it to a particular shape. Just prune to the rough shape that you want - ie pyramid, ball or any other shape, and the Bay tree will soon shoot out into new growth.
Further pruning can then take place by way of a light trim to bring the tree to the required shape.This is typical of the shapes that are popular with Bay Leaf Trees (Laurus nobilis). The long stem can be grown to this height over a couple of years. If doing this from an early stage with a small Bay tree, then allow the foliage to grow from the main stem in the earlier stages, as this will strengthen the stem. (It will also speed up the process of getting a long stem).
Once the stem is to the desired height, you can prune to top and then start the process of clipping it to shape over a couple of years. Make sure that the central stem is supported with a good cane until strong enough to support itself.
Important Point As the plant will be top heavy, make sure that the container is large enough to support it, and be aware of problems with it blowing over in high winds.
Mature Bay leaf trees can be cut back as hard as required early in the growing season. They will soon start to re-grow, and can then be restrained to a suitable size by regular clipping or pruning. Prune after the re-growth starts as describes above. In older gardens, very often small (?) kitchen bays were planted without any thought as to how large they can grow if left untended. I have personally cut back over ten such Bays - the larges being 10 metres tall and with a spread of around 8 metres!