A timer that sends an AJAX update request to the server according to a
time interval that you specify. The update request can then result in a
full or partial page update. You should avoid using this component with
enhanced lists.

Visualforce page:

 <apex:page controller=”exampleCon”>
    <apex:form>
        <apex:outputText value=”Watch this counter: {!count}” id=”counter”/>
        <apex:actionPoller action=”{!incrementCounter}” rerender=”counter” interval=”15″/>
    </apex:form>
</apex:page>
                    
Apex Code: 
          
public class exampleCon {
    Integer count = 0;
          
    public PageReference incrementCounter() {
        count++;
        return null;
    }
          
    public Integer getCount() {
        return count;
    }
}

Attribute:


interval  –  The time interval between AJAX update requests, in seconds. This value must be 5 seconds or greater, and if not specified, defaults to 60 seconds. Note that the interval is only the amount of time between update requests. Once an update request is sent to the server, it enters a queue and can take additional time to process and display on the client. 


Cheers!!!