What is the Statute of Limitations for Motion 664.6?

What is the statute of limitations for Motion 664.6? From everything I have read - and I am not sure if it applies to child support modification or not - (the statute of limitations applies - not if Motion 664.6 works for enforcing agreed upon settlements - I know that it does - although, I think it does - and I could be wrong!). Anyhow, from what I have read about Motion 664.6, it is a 2 years statute for a verbal agreement and a 4 yr statute of Limitations for written agreements. When does the statute of limitations begin? When the agreement was agreed upon? OR do the limitations begin the year the other party did not uphold the agreement?