How long should my battery last (when not in use)?

Get a Blue Seas or similar four position battery switch. It will let you select OFF, batter 1, battery 2 or both batteries. This four position switch will give you the ability to add another battery and select between the two or use both. Also add an inexpensive digital volt meter near the battery and one inside where it's easy to see. They do glow at night so consider that if it might bother you at night. You want to stay above 12 volts and many of us like 12.2 or so.