I have just done an experiment.
I have pretty much had my work lenovu laptop plugged in since November 2015 and very seldomly take it home on weekends where it is plugged in again.
This thread got me wondering if the battery life would be significantly worse then when it was bought.
I unplugged it at 11:28 this morning and I have been using it ever since its now 14:30 battery life is 64% estimated time remaining 5h15min.
So 3 hours of use with 5 hours remaining seems about right to me. I think most laptops have about 8 hours of use but stand to be corrected.
Will update just before 16:00 and see if anything drops dramatically.
You can see the battery capacity with a program called BatteryInfoView by Nirsoft.