Its completely pedal dependent and will vary quite a bit. You can use a battery up in a digital delay pedal in two hours. Or like in my Woolly Mammoth,it still reads over 9.4 volts and its been in there for 3 years.
Generally batteries last longer in analog effects, they also last longer in fuzz and distortion than in modulation.
I buy boxes of batteries, they come in 20 packs for about 10 bucks, they aren't alkaline but last ages in my analog pedals. I only use adapters on effects that require higher voltages.
