Battery degradation depends on your usual usage pattern. If you regularly let the battery discharge to near-zero, that shortens the lifespan of the battery significantly. Many experts recommend recharging when you reach 30-40% as a rule of thumb.
Batteries can also develop dendrites, which lead to sudden dropoffs in battery charge. These are unfortunately irreversible, and require replacement of the battery.
Also heat is bad on batteries, so if you have a case which prevents heat discharge, or play games while charging or any other number of scenarios repeatedly in the life of the 2 year old phone that too can shorten the life of the battery.
Sold my M9 last week for the lack of replaceable battery. I was not the original owner. No "warranty".
The phone was great, but it is 2 yrs from release and I didn't want to get stuck with a brick sometime in the 3rd yr.
ifix says it is a difficult task to replace the battery. I expect The phone may never be the same, like a wrecked car after repair. With a lack of users reporting results of a repair, I had to jump off.
Got a barely used Note 4, after my wife and friend replaced their batteries and said they would buy the phone again.