To maintain good hygiene when using plastic bottles, it's recommended to replace them at least once a year. This is because microorganisms can thrive in the tiny crevices and corners of the bottle, potentially leading to health risks. Although stainless steel bottles are more resilient, it's still advisable to swap them out every few years to prevent any potential bacterial growth. Glass water bottles are relatively safer and can be used for a longer period of time, typically 2-3 years, without the risk of ingesting harmful bacteria.