I voted for 45mins, although it does really depend on the cars. To me it is important to bring tyre choice and pit stops into it when it comes to tournaments/leagues. Having a good endurance setup can make all the difference even if you are not the fastest.
In the SHAC we did 30min Sprint race and then 90min Main race which was pretty good although it didn't really leave much of a pit strategie with the GTR class as the FXR had to pit exactly half way through the race to be competetive with the tyre choice, so no margain for error there.
For public servers of course anything more than 10-20 minutes is a waste of time unless it's a good server.
So to sum this up from my point of view:
Long enough to allow for different tyre/pit stop strategies