Demo time is 30 minutes. But it seems to let you continue to drive around the city without being able to enter races etc. Also, after the 30 minutes it starts to 'nag' you by switching to a screen prompting you to buy the game. But pressing a button takes you back into driving again.
After a while though, the button to take you back to the game changes to a 'Quit' button, which exits the game. I got about 47 minutes before I had to quit.
Re-running the game gives you a bit more time to simply drive about (no challenges though), and eventually, it forces you to quit again.
I have the PS3 version, and I'm not sure if I want to shell out for the PC version as well. The PC version does look better though, but probably because of the higher resolution I caan get (1920x1200). PS3 version already has the additional bikes as a free upgrade, so not sure I want to pay another £30 to get this on the PC.
Is a very nice game though