In some parts of the country, such as California, they can appear any time of the year. There are several stages to their life cycle. Check this website out.
https://entnemdept.ufl.edu/creatures/veg ... in_bug.htm
I guess they are a problem in brassicas (Cabbage, broccoli, cauliflower, etc.) which are generally a cool weather crop. SO, I would say you can expect them to be a problem from early summer until those crops are harvested.