Last chance to split?

When is the last week/month to split a 2-story bee hive in Sacramento/San Francisco, California area? Early/late August? Early/late September?

From what I know of that area, I think your climate is VERY near what we have here in coastal TX, with very, VERY few days in the year not reaching a high of nearly, if not quite, 50F. That said, it's never really cold enough for the temperature to be a grave threat to your bees, so you can "get away with" taking some very small colonies into the winter...so long as they have enough pollen & honey/syrup to get them through.
I don't know anything for a fact here, but I'd say you can safely split up until your bees start kicking the drones out of the hives, and then only have to stop for lack of drones to mate the emerging queen(s) (as long as you're willing to "baby" the resulting hives through the winter).

Thank you.

I make a split in the spring during the main flow with 1 to 1 1/2 frames of brood. As it gets later in the summer that number goes up. I will make a split in early September with a ripe cell as I have drones at least until the 15th of October. That split would have 5 frames of brood and bees, 2 drawn empty frames and 3 frames of honey in a 10 frame deep and I would be feeding 2-1 syrup at the same time getting them ready for winter. In a five frame nuc it would have 2 1/2 frames of brood and bees and 2 frames of honey. They have to have time for the queen to get mated start laying and the bees to organize their brood nest and stores before winter . The amount of stores goes up as you go north. I can easily winter in a single deep and winter a lot of 4 and 5 frame nucs.

Last year my hives had a queen out mating over x-mass holiday. She wasn't mated very well, but I was impressed.
If you make July-August splits it will help with the mite load.