I'm not sure there's any set period of time and I doubt they publicise it. I would guess it's an ongoing thing and you'd probably see a difference every couple of weeks. Google list your backlinks in your Webmaster Tools.
Some other options are: - surf to Yahoo site explorer - download the seoquake toolbar plugin for your broser. This tools shows also other interesting SEO info about a site.
withing each 10 days Google updating BL and also m using third party tool to check my BL( backlinkwatch)
How long is a piece of string? Trying to second guess Google. Tut tut. No, really. it appears to be as the above member mentioned - roughly every 3-4 months. However, where Google is concerned, this forever changes.
Google updates backlinks as soon as they find them. To know how many backlinks did google find .. you need to setup a google webmaster account and see them under "pages with external links". I am sure that you are asking about the "link:www.........com" list. There is no fixed time for the list to be updated and you'll never see some links on that list. My guess is that you'll see a link on that list when the link is still pointing to your website after some amount of time.
It depends on how often the backlinked page gets spidered. As soon as the spider finds it, you've got a backlink. As others have said, publishing and subsequent re-ranking probably occurs on an incremental basis. Google isn't going to perform a ranking re-calculation every time it spiders a new backlink.
I found a difference in the serps within a couple of weeks of starting a linking campaign to a back page. The pr hasn't yet changed but it's creeping nicely up the serps.