Don't ask your customers to do what you wouldn't do. For instance, would you wait an hour for tech support? Would you send a letter to get your statement adjusted? Neither would your customers. Make it easy for them to do business with you or they'll do business somewhere else.
This philosophy hit home with the owner of a large furniture store who had to wait an entire day for the delivery of his new computer system at his home. He asked for an approximate time of delivery, but all he got was "Sometime on the 12th." He was steaming by the time it arrived, and as a result, he vowed to never make his customers wait at home for more than a two-hour period. He even promises that his delivery people will call when they're within a half hour of arrival.