For best battery life you are supposed to do the initial charge at an amperage that does not exceed 1/10th of the battery capacity in amp-hours.
So for a smallish ATV battery of 5 amp hours the charge rate should not exceed 0.5 amps, hence the chart they gave you. Ideally it should take 10 hours to charge the battery at this rate.
If you go ahead and use the 2 amp charger anyway, the battery will still work, but you will cause some sulfating of the plates and the ultimate life may be compromised.
Assuming a very unsophisticated 2 amp constant-current charger, and assuming the typical 70 percent efficiency in storing charge, we would expect a 5 amp-hour battery to be fully charged in about 3.5 hours. Do not charge it any longer than that.
If the charger has a trickle or float mode, it will roll the current down as the battery voltage rises, and take longer to get to full charge. But in that case it should also limit the current to not damage the battery. Probably 4 to 4.5 hours should be the limit.
For a constant current charger, when the voltage with the charger connected rises to 13.8 volts or so, you are done. Don"t let it keep charging all night, or you"ll fry the battery.