Even if you only put plain water in your water bottle, you should wash it at least once a week. Other sources will tell you to wash it once a day, but who has time for that? Once a week is realistic. (Or, if it starts to smell like dirt, that’s also a good indicator that it should be washed.)
If you’re putting flavored water, juice, and other liquids in your water bottle, be sure to wash it more regularly.