Google can remember the pages for months on end. I would propose several strategies
- Create a custom 404 error page. Make the page look like a new page but with error information and your standard navigation
- Check your serp's - files with very important positions - dont remove them, you could lose your serp - keep a file here with the exact same URL and the same content.
- Build better pages to replace it and promote them
- Add a new sitemap.xml, add in the new files and the highest ranks pages (from your SERP check)
- Add sitemap to Google WebMasters
- Watch your SERPS until the new pages replace them
- remove the old pages - the custom 404 error page should take care of any others
You can also program your 404 page to detect where people have come from (E.g. search, other site). If you have a link on another site to a page you've removed, you can 301 it or have your 404 give the user a number of suggested pages. If the user came from a search to an old page, then based on the search you could direct them to the new or other appropriate page
Once your SERP's have been replaced with the new ones, remove the old files and update your sitemap.