That depends on how many you sign up for and what types of alerts you choose. For example, you may receive an alert about changes in a stock price several times a day, while an alert about a new Harry Potter book might come much less frequently. You can decide what alerts you want to receive, and can cancel them easily if you change your mind later.
To find new alerts and to manage the alerts you receive, sign on to AOL and go to AOL Keyword: Alerts. They are also available on the web at http://alerts.aol.co.uk.