a couple of months is no biggie. Just leave it in the box in the anti static bag... I've seen motherboards on the shelf for over 6 months with nothing wrong with them. I could see if you weren't going to use it for a year + then yeah maybe removing the Batt wouldn't be a bad idea...
Then again, if your not going to use your MB for over 6 months to a year... Who cares about the battery. Even if it were to Fail to leak acid all over the MB. No Biggie you MB is obsolete and you'll need a new one anyway!
BTW, I've seen really old POS 486's with Battery Acid all over them and take out the old battery witch was usually solderd into the board and clean it off and it worked just fine...