Hi. Yeah, I estimate three to four tenths for each level, with the F Ultimate. It depends on each combination of car or circuit, always with official content, or mods well made, such as for example Patrick's circuits.
Marbles and dirt will also form off-line as the track rubbers in. This will mean you have less grip if you go wide or more chance to lock-up if you make a move down the inside, for example.
The tracks are progressive so if you set it to low and put in a bunch of laps eventually it will progress to medium then high etc. Depending on the compound/type of tyre the track will also rubber in quicker.