Can't really tell in the photo, do you have anything pointing at the photoresistor? I think you may have better results with a light directly pointing at it, and possibly also shielding the photoresistor from excess light. (just a piece of black plastic around it should work fine)
Then you'd just have to calculate the time of each type of coin you're going to use.
This is all in theory, so I'm sure it won't be as simple as it sounds, but in THEORY it should work! 
There are ... I don't know the name of them... they're used often in Robots, to read the wheel speed using the same idea.. I believe it's what you're using, but the other side has the LED pointing directly at the resistor, so there aren't any issues with excess lighting.