I've got a pair of Wally World group 24s on mine. They're going on four years old in RV use and are still kicking. Keep them maintained and they'll last quite a while. Anytime you let the lights go dim, you've probably already hurt them. Generally, don't let them discharge more than 50%.
Also, you mention "backup". If you mean adding an additional battery, they should be as close as possible in age and size. If yours is a year or more old, get two and use the old one on something else. If you mean you don't already have one, you probably won't make it but a night or two at the most on one battery (depending on the size).
Also keep in mind, if you run your furnace for heat, that will put a major drain on batteries if it's run often. The furnace fan is probably the single biggest amp draw you have.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition