Also, how does this percentage change during the day? Is it different in developed countries than in developing countries?
Due to the design/nature of the Internet such figures are really hard/impossible to obtain. There will be a lot of variance between countries/regions in countries in regards to time spent online, and which hours are spent online.
Also definitions of Internet access are a little difficult to pin down. Is someone sending an email on their phone on the Internet? Someone streaming TV through the Internet 'on the Internet'?
Pingdom has seemingly quite good statistics from 2010:
They also list their sources as well so you can check them out individually. It states there are around 2 billion Internet users worldwide (2010). If I'm just going to pull a number out of my ass and say the average usage per day per person is 30 mins (which is a total guess) that would mean in a 24 hour period there are 48 1/2 hour slots, with an even distribution there would be ~40 million users online at any one time. If you +- a generous 25% margin for timezone/wealth differences and general variance, I think you could expect anywhere from 30 million to 50 million people online at any one time.
There's a lot of assumptions there though, but it seems like a reasonable starting point.