Once our winter is definitely over, you can trim at any time.
I do not trim every branch at the same time.
I do what is called "selective pruning," meaning that you
cut branches at different times and at different levels.
I don't want my hibiscus to look like a hedge
that's been cut flat and straight across.

Whatever you do, never trim more than about 20% of the shrub.
Actually, that goes for just about any plant.
Otherwise you run the risk of killing it if you cut too much..