I've dared to say 20... though it's unpredictable! Sometimes you ask a general question and you expect 30 opinions but get just 5. Than you ask a question that's more specific and there are your 20 opinions :D The best strategy I think is to ask the question when North America is "out of bed"... in Belgian time that would be around 5 to 6 PM. In the USA it's than between about 9 AM and 12 AM... so many will be online. And in Europe it's my time, around 6 PM so they're also still online. People will see question, opinions and comments in the live feed and that triggers opinions :D
Lots of factors I find a behaviour question on GaG posted around 9/10 east coast time will pick up a lot of interest maybe getting to 40/50 mark and then you get a few that scrape to 5 mark - I will average it out to 12/15 mark.