It's a brand new S5 watch. The battery was tested and found to be optimal. I was also able to reproduce this problem with S4. A friend of mine has S3 and is experiencing a similar issue.
Finally - the fact that reset helps for several weeks pretty much rules out a h/w problem - but rather points to a data issue.
I feel your pain. I initially upgraded form my s3 to a new s5. used it for a week it would barely last half a day even after turning off noise and always on features. Did resets and restores and new installs. nothing helped. first on 6 then on 6.1 for a day. I returned it.
Sadly i had reset my s3 because it was going to be turned in so it updated to 6.1 as well. Now that is having the same ****** battery life. Barely makes it 10 hours now when before i reset and updated it it would last a day and half easy.
This 6.x update is ****. my wife has not upgraded hers and its still going almost two days.
Very pissed