What is the deadline to file a counterclaim in California small claims?

The rule is specific rather than approximate. You must file and serve form SC-120 at least five days before the hearing date. If the plaintiff served you sixteen or more days before that hearing, the window extends to ten days instead of five. Every count here runs on calendar days, not court days.

If you've realized you're already inside that window and haven't filed yet, don't assume the option is gone. Ask the court clerk about requesting a continuance, a postponement of the hearing date, rather than simply concluding you've missed your chance to counterclaim.