Please disregard my vote, as I only looked at the time spent. Kind of silly to put the "Top 20" restriction. That limits the survey to the very few, and means most can't vote.
Difficult to answer as getting a top 20 position is very different from getting a top 10 one.
For a top 20, if I have a car with a nice setup 2 hours is enough ,but to go that extra mile in order to be around top 5 I need about 6 to 8 hours. But it really depends on the car/track combo, amount and quality of others players and all that.