Battery lifespan

I have an extra battery with a trickle solar charger.  If it's never used how long will it last?  It's only for backup on my pontoon.

I typically get about 10 years out of my batteries. As mentioned, keeping them maintained and fully charged year round is important. 

Tried starting boat motor 2 weeks ago and battery was dead. Switched trolling battery to get motor started. Looked at starting battery and dated 2013 from fleet farm. I bring my batteries in the basement and charged at least once. Works well for me!

My last batteries on my Ranger boat were 8 years old when I traded it in. There were 3 trolling batteries and a starting battery. All of them were agm and plugged into a dual pro charger after every use and all winter long. The boat was stored in an insulated garage with intermittent heat. 

My deep cycle  batteries would last 2 yrs. I do use it as an outboard starting battery but it is only 25 horse so I dont think there is any harm there. Now days I trickle charge it several time every winter and it makes them last about 1 yrs longer, so I get 3 yrs out of them and that does not seem very long at all. A guy that worked at Johnson Controls gave me a deep cycle  battery once and it lasted 4 yrs. It was one of the good ones. I wish I would have taken better care of it, by charging it through the winter here and there.  I always forget to check the water cause I thought those days were over, but I guess not.

proper care and maintenance will extend the life, or at least make you aware of any issues while you are checking water levels ( I always use distilled water ), any corrosion that needs to be cleaned off, charge level, etc .... I usually do this spring and fall

We’ll spoken Westy. Ya just have to roll the dice and keep track of your warranty until that expires. I always write the warranty expiration date on battery with a paint pen. 

That is a question that cannot be answered. Nobody knows how long a battery will last, a brand new battery could last 1 or 7 years. Nobody knows when they're going to take a crap.