Every month, a portion of your contact data becomes outdated as people change jobs, move up and down the organizational ladder, etc. To learn more about data decay, check out this post by Donato Diorio. To quickly summarize, the rate of data decay in your CRM and marketing automation platform depends on two things, (1) the state of the economy and (2) your industry. When the economy is good, contact turnover is relatively low. When the times are tough, there is a higher job turnover. Certain sectors have lower-than-average job turnover, while others (like the high-technology sector) has much higher job turnover.
In a year, depending on the state of the economy and the industries represented in your CRM database, as much as half of your leads and/or contacts can be rendered useless.