You must file an application with the Court to modify child support. In New Jersey we have a 24 day motion calendar. I am sure that your state has some similar procedure. Once the application is filed, it will be scheduled with the Court and after the hearing the Judge will enter an Order. In NJ usually the date the application is filed is the date a Judge will make the relief sought in the application (if it is granted) retroactive to. You can call the Court and get the rules for your state.
How long does it take? As quickly as you get it done. Unlike modifying parenting plans, modifying child support is pretty straight forward. If child support has not been modified in over two years, then you can ask for a modification at any time. If it has been modified in the last two years, you need to show a "substantial change in circumstances."
My suggestion: read over the WA Law Help pamphlet at http://www.washingtonlawhelp.org/documents/1487413814EN.pdf?stateabbrev=/WA/. Draft on your own, or have an attorney draft, a petition and summons to modify child support, along with a citation for a hearing, a declaration showing your new income, and a revised child support worksheet. The nuts and bolts of figuring out the worksheet calculations can be overwhelming, so help is definitely advised.
Any answer given is an opinion based on facts presented and does not constitute legal advice. www.joshuapops.com