It takes 7 years, 2 months, and 14 hours until you go bald. It's called the "7-2-14 ratio" and it is found commonly in nature. Using this ratio, doctors can also predict how ticklish someone is going to be, whether or not the subject will have a beard thick enough to have to shave against the grain, and whether or not a female subject will break out if she attempts a Brazilian wax job. The clock starts the minute the subject first makes a posting on hairlosstalk dot com, and begins counting down.
Sorry, I was just playing around to add a little humor to the board today and that was rude of me... so I am editing this posting to add this paragraph to give you a better answer. The fact of the matter is, one really cannot tell how long it will take. It depends on your individual genetics and a lot of other environmental and hormonal effects.
Some people lose their hair quite quickly, and all of it. However MOST people lose it slowly, and in pronounced stages throughout their lives. As for me, I first thinned in the temples very quickly, but then it stopped for several years. Then, the temple baldness got really high on my head in what seemed to be the course of six months, and then it stopped for several years. Then, it started to thin again just last year... and that is when I started treatment. It is really hard if not impossible to give a definitive answer to this. Your best indicator, and it is not at all 100% reliable, is to look at the hairloss patterns of your dad, uncles, grandfathers, etc.