Not many... about a million hours of driving which is not even one day for the USA cars. Here is an interesting hydrogen article with the source at the end:

"Consider a mid-sized car, traveling at 60 mph down an Interstate Highway. It is well known (and easy to calculate) that roughly 40 actual horsepower is needed to maintain a constant speed. A horsepower is equal to 2544 Btu/hr, so we are talking about 102,000 Btu/hr of "actual" energy/work. For an hour's driving, we would therefore need 102,000 Btu of output energy. (A gasoline engine would use maybe 3 gallons of gasoline during that hour's driving (20 miles/gallon) which actually contained about 378,000 Btu of energy, but the engine/car efficiency is only around 25% to create the 102,000 Btu of output work.) Consider now that a cubic foot of Hydrogen (not compressed) only contains 319 Btu per cubic foot. That hour of driving would therefore require over 1,000 cubic feet of the Hydrogen.

We can say this same thing in terms of "gallons". A gallon of gasoline contains around 6 pounds, and has 126,000 Btus of energy in it. A gallon of hydrogen (gas) only contains around 40 Btus in it. Quite a difference! Instead of a two cubic foot gasoline tank (15 gallons) in your car, you would need a tank more than 3,000 times bigger, over 6,000 cubic feet, for the equivalent Hydrogen! That's a little more than TWO standard semi trailers (8'wide x 8'high x 45' long or 2900 cubic feet each). Pretty big gas tank!

Well, that is obviously not going to happen! So, the many ongoing explorations into using Hydrogen as a fuel always involve carrying HIGHLY COMPRESSED Hydrogen in very thick, heavy tanks. If you have ever seen the kinds of tanks used for the Oxygen for a worker's oxyacetylene cutting torch, that's the kind. Such tanks can hold Hydrogen at around 100 times atmospheric pressure, or 1500 PSI, an extremely high pressure (or even higher) (We will later mention even higher pressure hydrogen tanks at 3000 PSI).

Well, at 100 times atmospheric pressure, the Ideal Gas Law tells us that the Hydrogen would now only take up 2900/100 or 29 cubic feet. That works out to around 60 of those (fairly large) high pressure storage tanks (to match the effective capacity of the 15 gallon gasoline tank.). Each tank is very massive to withstand the very high pressure, and each weighs nearly 100 pounds empty. (And around 1/4 pound more when filled with Hydrogen!) So the normal American car which presently weighs around 2800 pounds would have around an extra 6,000 pounds added, so the vehicle would now weigh more than three times as much as current cars! (This tremendously affects acceleration, handling and other performance, and it would be like that car pulling a huge 6,000 pound trailer behind it." exerpt from http://mb-soft.com/public2/hydrogen.html