What is my deadline to sue over an unpaid business invoice in California?

If you're working from a written contract, Code of Civil Procedure section 337 gives you four years from the date of breach to file suit. If your agreement was oral rather than written, section 339 shortens that window to two years instead.

"Breach" in this context generally means the specific date payment became due and simply wasn't made, not whatever later date you personally decided to stop pursuing the debt informally. If you're dealing with what's called an open book account, essentially an ongoing running account of charges rather than a single discrete transaction, that has its own four-year limitations period that runs from the date of the most recent entry on the account.