No, it’s not linear. Some people can get a full day of use with a battery health of 77%, others complain of charging twice a day at 84%. What matters is whether your use pattern changes. When it does, it’s worth looking at the battery health. Some people stay at 83% for 6 months or more, just to annoy you at not ready for replacement yet despite barely getting through 6 hours. If it’s below 80% (79% or lower) you can get a battery replacement in the US for $99 (used to be $79, but like many things, Apple jacked up the price). You can then decide if you want your dump $99 on a fresh battery or towards a new watch.