The battery typically can go through 1000 full cycles before falling to 80% capacity (health.) Apple doesn't deem it to be a failed battery until it reaches this point, but as you've noticed on your phone, you probably feel like your battery is failing you now. I used to see this with some of the older iPhone models, but haven't noticed this as much with newer models even at the same battery health in the mid 80's.
For future: Keep the phone within 20-80% charged as much as possible. If it sits on the charger all the time, that can hasten its demise. The phone has some smart features built in to protect the battery, but it isn't perfect.
Regardless of the cause (6S battery defect, overcharging, or just length of time) you have 2 options:
1) Get a battery replacement ($49)
2) Get a new phone (wait till October if you can for new models)