That may vary from site to site. Basically, it depends on how often a search engine's crawlers visit your site.
The next logical question is "how do i get my site crawled?"
One way to increase crawl frequency is by increasing the number of sites that link to you...which you should always be doing anyways.
I've also read that some people are having success getting new content crawled and index by including a link in a Twitter post. Search engines are constantly crawling the Twittersphere looking for real-time content. The theory is that including a link in twitter will get a search engine to notice that page faster. I see how it would work for new content but I can't guarantee results for existing content. Worth a shot, tho!
Hope that helps! Just let me know if you have any follow-up questions. Thanks!