I have no idea how long it takes the pros, but they seem to pick up on new
tracks surprisingly fast. Myself, it usually takes me about 20 laps to
learn a track, then it takes me forever to really get the most out of the
track. Yes, there is always room for improvement, after those first 20
where improvement is dramatic, lap to lap, then we start talking about
shaving fractions of seconds at a time. These fractions can add up to many
seconds over time, but it takes longer to shave them off. Progress with a
track works like a logarithmic curve. Experience with a track is a big
factor with doing your best, for sure.
Keep in mind that before the individual tracks it takes hundreds of laps on
many tracks to refine your driving technique which is a key stepping stone
to doing well before attacking a new track. So general experience with the
sim/car is crucial.