How Often Should Reports Be Tracked?
The answer, of course, is that it depends. It depends on what changes have been implemented, the reports being tracked, the industry, the company infrastructure, and more. Here are some general rules to help get you started.

What Changes Have Been Implemented?
The main driver behind creating a schedule to check the key reports should be any marketing or content changes that have been made recently. Change is the main reason for checking the reports, with the reasoning based on the difference between a metric and a measurement. Saying that somebody is 5 foot 7 inches tall is just a measurement. Saying that somebody is short is a metric. Metrics are measurements that are given meaning through comparison.
For instance, that same person would be considered tall in Japan. Is your shopping cart abandonment rate good or bad? These are relative terms, and are given meaning by comparison to other companies or the changes that occur over time. No matter what your shopping cart abandonment is, hearing that it has gone down by 50% is good news!

New CPC Campaign?
When you have implemented a new major Google AdWords campaign, it is generally advisable to check the appropriate reports once a day for two weeks to a month. For smaller campaigns, checking once a week for a month is appropriate. What qualifies as a 'major' or a 'small' AdWords campaign depends on the company. For a company that normally has 500 AdWords Ad Groups, adding another 10 Ad Groups may be a small change.

Launched a New Email Campaign?
A new email campaign is different because of the lag between sending out the email blast and everybody checking their email. We have found that checking the reports three days after the email campaign was sent out captured most of the changes from the campaign.

Standard Maintenance
Standard maintenance, like all the others, really depends. Some industries move rapidly and experience more change, so they need to check more often. Some companies are just beginning, so they need to check more often. The objective is to check often enough to catch something and correct it before it becomes a problem. A good way to assess how often you should check for standard maintenance is to ask yourself how long it generally takes to correct a negative trend once it is spotted, and how often you discover these negative trends currently.
The general rule of thumb is to check once or twice a month. This should enable you to catch anything negative before it becomes a large problem and should keep you up to date with what is current.