The buying limit of Grand Exchange items is restricted to a certain quantity every 4 hours. This timer is based on the first item bought in the 4-hour slot. Regardless of the spread in time at which more items are bought after the initial item, 4 hours after the first item is bought, the buying limit is completely reset.
The following is a list of all buying limits for items purchasable from the Grand Exchange:
The following items have no known GE limit. One may exist but simply hasn't be found or documented yet.