The answer to 'when to replace bathtub' is that you should typically replace a bathtub every 15-20 years, or sooner if it shows significant signs of wear and tear. Bathtubs are exposed to a lot of daily use, moisture, and potential damage, so they have a limited lifespan before they need to be replaced.
Some key signs it may be time to replace your bathtub include cracks, chips, stains that won't come out, or a worn, uneven surface. If your bathtub is starting to look old, outdated, or is difficult to clean, those are also good indications that it's time for an upgrade. Replacing a bathtub can also be a good idea if you want to change the style or layout of your bathroom.