Mine is nearly 5 years old this year and still seems to be ok (touch wood!) I'd say yours should be good for another few years, especially as I'm guessing it's a bit warmer where you are?
As for the occasional use, it might be a good idea to wire up a trickle charger, the sort you can get for vintage cars and motorbikes, something like this:
http://www.argos.co.uk/static/Produc...er/7405392.htm