About 1 - 2 inches, depending on the bottle. If you use a bottle filler you can just fill it to the very top, then when you pull the filler out of the bottle the correct amount of air space will be left.
The crucial point is to make sure the beer is up into the neck of the bottle. If you leave airspace down into the body of the bottle there will be too much volume for the beer to carbonate, and you'll end up with flat oxidized swill.
[/I] Up Next - Hobgoblin
After That - Czech Pilsner
Primary - Humboldt Hop Rod (4/24)
Primary - NOT Wheat AG SNCA (5/5)
Conditioning - SNCA Clone (3/3),