The number of entrants very much determine the true lenght. The rules cater for a maximum 150 drivers (last I checked) and you can roughly calculate the amount of time by

Per heat
( Leadin + Warmup + Duration ) * ( Drivers / CarsOnGrid )


Per meet
Rounds * ( Leadin + Warmup + Duration ) * ( Drivers / CarsOnGrid )

With a day comprising of 400 minutes of racing ie 2 x 200 minute sessions, the ideal meet value would be 1200 minutes for a 4 day and 800 for a three day.

If the notion is to attract more drivers, the dilemma is that it'd be more social to shift things earlier complete Sunday early and open the track for some post social fun. That then makes it harder for those with work commitments.