Personally: At least 45 minutes, that way it isn't like a sprint race and people will stay away from reckless overtaking to that topspot Keep their cool a bit longer and produce awesome fights
Also I do tend to gain more experience on a track with longer race (duhh) and that helps me scrape off tenths to even seconds of my PB.
Long enough for strategy and pitstops to be involved for me. As an example, a race with 65min driver stints, but that is just long enough that you can eak out 1 less pitstop if you get the stints right, or having to double stint tyres on limited sets. As close to the real thing as possible please!
EDIT:
I also think the thing here with the poll results is probably down to the wording of the question. 'online race' suggests pick-up-and-play casual racing, in which case, 15-30mins is probably right. Whereas if this is meant to be feelers for future leagues, you may want to reword the question..