How often do the batteries in solar garden lights need to be replaced?

The battery replacement cycle of solar garden lights mainly depends on the battery type, quality and use environment. General:

Lead-acid batteries: usually need to be replaced within 3 to 5 years, as they have a cycle life of about 300 to 500 charge/discharge cycles.
Lithium Iron Phosphate Batteries: may offer a longer service life, around 8 years, as they have better cycle stability and high temperature resistance.
 

Lithium-ion batteries: may last longer, up to 10 years or more, under ideal conditions, but this is also affected by frequency of use and maintenance conditions.

The actual replacement time also needs to consider the frequency of use, charging efficiency, ambient temperature and other factors. If you notice a significant reduction in the lighting time of the fixture, or if the battery charging rate slows down, it may be a sign of declining battery performance, at which point you should consider checking or replacing the battery. Regular maintenance and proper storage (e.g., during extreme cold or hot seasons) can also extend battery life.