Can anyone tell what exactly makes a search engine index your webpages daily ? Is it daily new content and lots of IBL ?

I ask you because I am building a website with many job ads. If I make links on the site useing one display_ad.php file with parameters for displaying the ads from the DB, will the SE index all those pages ? 'Cause if they index only once a week there is no reason to finish this project cause the ads will be out of date.

Any ideea will be appreciated.

Thanx.