How Fast Does Gravity Fall?

I'm looking to use Kraeusening to carbonate and clean up a cream ale. I'm using a spreadsheet tool I found of Braukaiser.com that tells you the amount of wort and gravity that should be added to obtain my target Co2 volume.

In my case i need 1 gal @ 1.039 to acheive 2.7 vols of Co2. My starting gravity is 1.056. I'm using WLP080 for my yeast. My question is:

How long should I expect 1 gallon of wort to ferment down to 1.039? I know if I overshoot I can add sugar to compensate. I just don't want to end up bottling my beer at 3AM because that's when the magic number hits. Is this at all predictable?

Dan

Pretty hard to predict in my opinion. I would think somewhere between 12- 24 hours of active fermentaion though, but thats just a guess.

Once you get signs of active fermentation you could always take a reading then check again in an hour to see how fast your gravity is falling each hour and do the math. Not sure how accustate this would be either though since as your yeast multiply the gravity will fall quicker.