The standard amperage, or charge rate, is typically 1C, or stated differently, equal to the pack size. If you have a 2000ma battery, 1C=2A. If you have a 5500ma battery a 1C rate would be 5.5A. In your 2000ma example you listed a 1/2C rate of 1000ma (1A) which theoretically doubles the charge time from what is required in a standard 1C charge rate. In practice that time varies somewhat. With very large battery capacities, for example a battery of 10,000ma - 22,000ma, it may not be possible for some chargers to charge at a full 1C or higher rate. The battery could still be charged, but at a reduced rate.
There is nothing wrong with using charge rates <1C as long as charger cut off times are adjusted to permit a longer charge time. Lower charge rates will prevent heating of the battery during the charge cycle. Charging a battery at rates >1C can have detrimental effects on batteries. If you desire to charge at a rate >1C you should consult with the battery manufacturer, or review battery documentation/labeling, to establish that a higher charge rate is safe for a particular battery, and what the maximum allowable charge rate is. I'll suggest that anyone considering charge rates >1C obtain and use battery temperature sensing devices and monitor them closely. If your charger does not support the use of such tools, limit the charge rate to 1C or less.
Chargers that have adjustable charge times are often shipped with a default setting based around an approximate 1C charge rate, which could terminate the charge cycle before the process was completed if charging at rates <1C. Consult your charger's instruction manual for more information on cut off times.