It's hard to give an average, since a race to 9 can be over in 20 minutes of someone wins 9-0, or can go two hours if it goes hill-hill (I've seen a 3-hour race to 9 before). An average in between the shortest I've ever seen, to longest for races to 5, 7, and 9....
Race to 5 average- 40 minutes
Race to 7 average- 60 minutes
Race to 9 average- 80 minutes
It also depends on the calibur of players. These averages are for good players. C players and below, throw on an extra 30 minutes to each IMHO.