The answer to 'when to replace toilet' is that you typically need to replace your toilet every 15-20 years, or sooner if it's showing signs of wear and tear. Older toilets can become less efficient, develop cracks or leaks, or simply not function as well as newer models. Replacing an old toilet can save you money on water bills and provide a more comfortable, hygienic bathroom experience.
If your toilet is constantly running, has a cracked tank or bowl, or is flushing poorly, those are good signs that it's time for a replacement. You may also want to upgrade to a more water-efficient toilet model to reduce your water usage and environmental impact.