There is no hard and fast rule about how long it will take employers to respond to you with a job offer (or a job rejection). The hiring process can vary from employer to employer, the type of job you apply for, and the industry in which you work. You may receive an offer in a day or two, or it may take weeks. According to several hiring managers, requesting 48 to 72 hours is perfectly reasonable.
Charlie Nelson, vice president of SmartRecruiters, works with hundreds of recruiters and has hired hundreds of people himself. The right people must approve new hires, often in a very specific order in the organization's management chain, and decisions are made while waiting to receive the appropriate approval in order to move the paperwork to the next level.