Depending on the make up of your site, the url you are searching for, and the number of pages you are looking for, a filtered scan can indeed take some time to complete.
The filtered scan essentially will crawl your entire site looking for pages that match the url pattern you entered in the scan dialog. It will do this until it has found 20 pages (the page number you have selected). Once it has found 20 pages that match that pattern, or there are no more pages to crawl, it will complete.
The scan may be taking a long time because VisibleThread Web has to look hard to find 20 pages from the site that match the filter you have entered.
Filtered scans work this way to ensure accuracy, essentially to ensure that VisibleThread Web does indeed find all pages from your site that match the filter.
If you know the URLS of all the pages you wish to scan under your site, you can choose the third tab under the 'new scan' diialog and enter the urls there directly. This will be much quicker than using the Filtered scan approach.