What is the rule of trimming geraniums in the fall/winter to have maximum flower the next year please? I heard you should cut geraniums all the way down in the fall/winter.

Thanks

Pelargoniums? The Pelargoniums Database
or Geraniums? The Geraniums Database

I trim my geraniums way back to about 2-3 inches from base, the pelargoniums get pruned back throughout the year to keep them full, in the winter I usually cut back by about a third. (for those that can leave pels out year round)

@Calif_Sue This is what I have? Which one is it please? Pelargoniums or Geraniums

It's a Pelargonium but their common name is geranium, I know, confusing.
I just trim them back to shape them and remove any long scraggly stems.

How deep can I trim them back please? way back 2-3 inches from the base? Also when should I trim it please? December?

Where in California are you? Do you get frost?
Like I wrote above, I pinch the stems back all year, removing spent blooms and scraggly stems to maintain fullness.
I then wait until spring after danger of any frost and trim back by maybe 1/3rd which is when I add some fertilizer and a layer of compost.

I'm in southern CA, zone 10. I don't think I have frost

Just pinch back year round as needed. It encourages new growth. No need to cut back severely in your climate.