The answer to 'when to replace bathtub' is that you should typically replace your bathtub every 10-15 years, or sooner if it becomes cracked, chipped, or damaged. Bathtubs can wear down over time due to regular use, exposure to water, and changes in home decor. Signs it's time to replace your bathtub include visible cracks, chips, or stains that can't be easily repaired, and if the tub is no longer draining properly or holding water well.
Replacing a bathtub is a significant home improvement project that is best handled by a professional plumber. They can properly remove and dispose of the old tub, and install a new one that is properly sealed and connected to your plumbing system. Attempting a DIY bathtub replacement can be risky and lead to water leaks, so it's important to have an expert do the work.