Why the landlord must act right away
Because sewage is a health hazard, a landlord must address a sewage backup or overflow right away — not within days. This is one of the most urgent habitability problems, and it usually calls for an immediate, emergency response.
There is no fixed number of days in the law, but the "reasonable time" a landlord has shrinks dramatically for something as serious as raw sewage in a home. A landlord is generally expected to arrange emergency cleanup and repairs promptly, even after hours. Every hour of delay increases the health risk and the contamination.
Reporting the problem in writing and noting the time helps establish when the landlord's clock started. Keep copies of your reports and any emergency requests. If your landlord lets a sewage problem drag on instead of treating it as the emergency it is, a free review can help you understand your options.
This is general information about California tenant rights, not legal advice. Every situation is different.