If could begin to run as soon as the judge enters an order for child support. Therefore, if the Petitioner asked for temporary/interim orders, then the child support started as soon as that order was entered. It also depends on whether the judge made the support payments retroactive. You need get a copy of any final order and any temporary/interim order. The order will tell you when it started and if it was retroactive (if California allows for retroactive. I am not licensed in CA. But you should start there. You can also request a copy of the payment history from DCSS.
The information provided is not advice but a legal perspective and you should schedule a consultation with the lawyer of your choice.
The order will be effective on the date it was signed by the judge. In addition, the court could have made the order retroactive the the date of filing. This would account for the significant arrears and the interest running. At this juncture, if the judgment was in fact entered by way of default, you may want to consider filing a motion to set aside the default judgment based on lack of due process (notice). It would be wise to consult with a local Child Support attorney if you decide to go down that road.
As soon as it is ordered to start per the judge.
My name is Stephen R. Cohen and have practiced since 1974. I practice in Los Angeles and Orange County, CA. These answers do not create an attorney client relationship. My answers may offend I believe in telling the truth, I use common sense as well as the law. Other state's laws may differ.. There are a lot of really good attorneys on this site, I will do limited appearances which are preparation of court documents it is , less expensive. However generally I believe an attorney is better than none.