Personally I would be very wary of using the now() statement in a dashboard, as this will interrupt QlikView doing other processing and be a resource hog. Where I have done an auto advance before I have used a HTML page with the following on it:
This advances to the next page after 60 seconds, and the browser looks after the timer - so it can not impact on the QlikView server. There are a number of HTML files each with a different URL in the iFrame - using the sheet parameter to the opendoc file allows for progressing through tabs in a single document - but you could equally go to tabs in different documents this way.
Not sure how you would do the calculation of the duration in an HTML page though - you could do it with ASP but you would have to pass in how many pages it was enumerating.