It depends on the plane, situation, etc... the F6F in AH has, for example, 250 gallons of fuel for a weight of ~1500lbs (using 6lbs/gallon). The aircraft, according to AH help pages weighs ~12,500lbs so the fuel makes up something around 12% of the total weight. Something like the F4U-1 has 361 gallons and around 18% of the weight of the aircraft is fuel. An La7 has only 122 gallons, only 10%. Thus, something like an F4U-1 tends to show more difference in handling at higher fuel loads.
I seem to remember looking at some different EM diagrams at one point and it appeared that 25% fuel made a difference of about 1 degree/sec. This was a very rough guesstimate at best though and was using the F6F-5 as an example. I think there was a similar EM diagram for the P-47D11 for a few different fuel loads. You might want to look through Badboys files on the
http://www.netaces.org site.
I know the 190A8 is an aircraft where I can really feel the fuel load difference, with heavy fuel it just seems to give more of a sinking feeling.
