You can do the math on it:

battery capacity in mAh / charge rate in mA = hours to charge.

600mAh / 2000mA = .3 hrs, or 20min.

To calculate what multiple of "C" you are charging a:

charge current / battery capacity

2000mA / 600mah = 3.33, on the high side.

If you combine all that together, you can see that the rate expressed as a multiple of C is really the same as saying how much time it will take to give the pack a full charge. C is 1 hour, 4C is 15min(or 1/4hour), C/10 is 10 hours.

As FHH said, when given the choice between two rates, take the lower one, your batteries will thank you.

(btw, charging isn't 100% efficient, so a charge calcualted to be 1 hr long will actually take a little longer)