Question
In southern Calif., what months should pine trees not be trimmed?

Answer
The winter is the best time but they can be pruned any time. If pruned in the spring they have a tendency to bleed more since this is the time more sap is moving from the roots to the foliage.

It is not how much you cut but how much you leave. A pine needs at the least 1/3 of the length of the tree to be in foliage (crown). More cut off then that will cause the tree to suffer in growth loss and could over time cause it to die back. I would prune just enough to get the branches off what ever you are trying to do --above the heads of folks walking under the trees or off buildings etc.