As long as you want, or until your arms and legs get tired
Mountain bike, helmet and possibly body armour depending on how adventurous you are (or bad you are!).
Mountain biking started off by having rugged bikes that you could ride off road. Things have dramatically developed since then and you can get all sorts of mountain bike depending on what you want to do with them.
You can get a mountain bike without any suspension. Ok for use on the road, but if you go offroad the bumps will take their toll on your arms and legs.
Hardtails just have front suspension. These are great for general freeriding. The front suspension on the mountain bike takes the shock out of some of the bumps and is easier on your arms.
Full suspension mountain bikes have suspension on botht eh front and rear wheels. These are great for downhill runs. One downside though is that when you are pedalling, some of your pedalling power is lost to the rear suspension bouncing up and down.
Cool Mountain Bike Action in the City
Cool North Shore Mountain Bike Action in the Countryside
Get outdoors and get some of this action! Don't forget your body armour!