I’m more familiar with Phonak, but I’d bet all the manufacturers have similar battery lifetime expectations. For Phonak, rechargeable hearing aids are expected to deliver a “full day” charge for a lifetime of at least 6 years: Learn more about Phonak rechargeable technology | PhonakPro
I’m not sure if that’s explicitly promised in the warranty. If you have, say, a 3-year warranty on the aids, you might want to carefully evaluate them to see if there is any need for an in-warranty service right before the warranty runs out. I think typical service procedure is to just swap out the battery and electronics.