I've had mine in for a month, no signs of going yet. And I know that the same battery was in the pedal a few weeks before I bought it, so give it two months.

Just unplug it when you're not playing.

Also, if you don't wanna buy new batteries you can get DC adapters that clip onto a 9v battery terminal, so you don't need to mod the muff.
"You can practice to attain knowledge, but you can't practice to attain wisdom." - Herbie Hancock
^Indeed, I've got one of those and it works great. As long as you've got a power supply and possibly a daisy chain if you've got other pedals.
"Breathe, breathe in the air
Don't be afraid to care"

Fender Strat/Tokai LS80>few pedals>Orange Rocker 30
Well my Boss GE-7 equaliser takes about one 9v Battery in 2 months, if that's any help.