It will be fine, but I would occasionally unplug it and let the battery drain a bit, then plug it back in.
I have a 2001 iBook, and in 2003, I let my parents use it while I was in Afghanistan.
It was plugged in for almost a year, as my parents, like most people back then, did not have wifi, and their cable modem was plugged into the laptop using ethernet.
The Laptop stayed on and plugged in. Not unplugged once, until I came back.
As soon as I unplugged it, the laptop died. I plugged it back in, and it started up fine. But, the battery wouldn't hold a charge at all. The battery was fried.
Apple used to recommend discharging the battery of their laptops every once in a while, but if you go on their laptop battery support site, they don't give a recommendation anymore.