I'm sure the accuracy will be somewhat limited. Fuel-injected cars will have a fuel pressure regulator, but I wouldn't expect it to be an extremely high-precision device (and your fuel pump will have limited pressure/flow as well), so (as audioguru said) you would probably end up with variations in fuel pressure with other factors, and thus your duty cycle alone would not tell you the exact amount of fuel you were using. How closely you could predict it would depend on how good your FPR regulates, and/or whether you try to model its imperfect performance if it is too far from ideal.
Also, in response to someone else's idea, a flow meter on the fuel line is not a great solution either, because with a FPR you have both a feed line and a return line, so you would have to accurately measure BOTH flow rates and find the difference. I think this would end up introducing more error than the other method, not to mention being much more complicated (I don't think a gasoline-safe flow meter that will not interfere with your fuel system's operation is going to be a very common item)