how long does battery last?

That really depends on how you treat it. My 2011 MBA turned 3 in December and has just over 100 cycles since I use it plugged in more than I do on battery. But I make sure to unplug it at least once a month and use it that way until the indicator is red (about 30 minutes left). Between age and Yosemite (which did take off about 10 - 15 minutes a charge) I get about 30-40 minutes less than I did when it was new.

apple recommends that it be used on battery a minimum of once a month not until it totally discharges, completely discharging the battery is very bad. I'll plug it back in when I get the warning I'm on reserve power. apple claims the battery should still charge to 80% of its original charge after 1000 cycles - a cycle being a full discharge and charge (using it on battery until it has halfway discharged and charging it twice would also be one cycle,) I have a 2009 MBP that was getting about 80% of original after more than 4 years but began to drop to about 65% in the next 18 months. It didn't get 1000 cycles but 4 years impressed me.