according to this site 1.5 to 3 years should be about the life which would tie in with mine when i had my lappy.mine was in my lappy for 2.5 years with charger plugged in constant when lappy was on.(should have bought desktop i know)click here
Leaving them constantly puged into the mains certainly reduces the life of them. My first laptop the battery lasted about a year. Now when im on my Mac Book I only have it plugged into the mains when it needs charging.