I never quite finished it because my 360 died on me but yeah, thirty hours sounds right. I agree though, don't rush it. They had some brilliant design ideas too. I love that since it's built on the GTA engine which lets face it isn't great when you need to be precise that they put in 'hey you look frustrated, skip to the fun?' option. After the first few missions you don't have to drive!
What they needed though is a Partner stay in the car option. You'll find out.