The answer to 'when to replace septic tank' is that most septic tanks need to be replaced every 20-30 years, depending on use and maintenance. Septic tanks are critical for properly treating and disposing of household wastewater, so it's important to replace them when they reach the end of their lifespan. Signs that your septic tank may need replacement include slow draining sinks/toilets, sewage backups, and standing water around the septic system.
Proper septic tank maintenance like regular pumping (every 3-5 years) and inspections can extend the life of your system, but eventually all septic tanks will need to be replaced. Replacing a septic tank is a major project that requires the expertise of a licensed septic contractor, so it's best to have an expert evaluate your system and provide recommendations.