HI @sgoldwater - how long is a piece of string? It depends mainly how it is used and it's environmental conditions, also how frequently you charge it.
My Sense is 3 and a half years old and fine, but I generally only charge every 5 days or so.
You only need replace it once it doesn't work, and that is after doing the correct troubleshooting because simple things may make it play dead when it is fine, for example How to check charge cable - Sense/Versa 3 won't charge, won't start or blank screen
If it gets wet it can fail and when charging check it doesn't get hot and if so stop immediately.
Author | ch, passion for improvement.
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