battery dead...and questions

I reviewed the hand book, say during winter store time, you have to check and charge the battery at least once a month.



But after 3 weeks when I went to storage and check the bambi, the battery's already dead.



My bambi 2012 model has 2 batteries installed by the 1st owner. I bought it this summer



Have 3 questions:

1. Usually how long your battery can stay from drain off?

2. How many years do we need to change battery?

3. Will the dead be harmful to the life or capacity of the battery?



Thanks!

Eddie