What is the statue of limitation on past due invoices?

In California, the statute of limitations on an oral contract is 2 years.

In California, the statute of limitations on a written contract is 4 years.

In California, there is an additional statute of limitations in Code of Civil Procedure section 337(2) as follows:

2. An action to recover (1) upon a book account whether consisting
of one or more entries; (2) upon an account stated based upon an
account in writing, but the acknowledgment of the account stated need
not be in writing; (3) a balance due upon a mutual, open and current
account, the items of which are in writing; provided, however, that
where an account stated is based upon an account of one item, the
time shall begin to run from the date of said item, and where an
account stated is based upon an account of more than one item, the
time shall begin to run from the date of the last item.

More facts would be necessary to see which of these statutes apply.