How long to walk up the Mutianyu Great Wall from the car park?

Hi, how long does it take to walk from the car park to the top of the wall? My tour guide is trying to get me to buy a 180RMB pass for the cable car/per person, and I'm not able to afford that for 4 people. I dont want to buy it, and he is saying I wont have time to walk...
I have a 9hr stop over in total and he is picking us up from the airport. Is this too optimistic? Should I just take the train down town instead? and look at tiamen square?