If your battery life is about 2 hours then I would guess that a year ago it was about 2¼ to 2½. Lithium-ion batteries lose capacity over time naturally, and they lose it a little faster if stored fully charged and much quicker at higher temperatures.
Leaving the charger plugged in won’t really affect it as much as some people think since the power management will cut the charger off when the battery is full, but it also means that the battery is fully charged all the time so it will lose capacity somewhat quicker.
However, that loss pales in comparison to the losses caused by calender age as certain parts of the battery interact with the air, and the temperatures that they typically operate at (remember, the battery is typically fairly close to the CPU and heats up a little on it’s own anyways due to current draw and internal resistance) will kill it even faster.
I believe that typical loss for a LiON battery is in the range of 20–30%/year under normal use and as bad as 80% in three months if you store it fully charged at temperatures normally encountered in a parked car during the summer (~140F).
@sjmc1989 There is no way to undo the damage. Sorry :(