How on earth are we supposed to answer that? How long is a piece of string? The older the link the better it is.
Normally for google you have to wait for next backlink update and that usually happens once in two or three months. Here the qulaity of the back link, age of the backlink and how relevant is that ? matters a lot for google serps. Yahoo and MSN normally starts counting fast as soon as the pages are cached. Make a long term plan for top quaity links rather than short term.
Google takes links into account straight away. They then increase the weight of the links gradually over a period of months/years.
yeah I noticed in Google webmaster tools it took a few months for the links to show. Think it depends on how many users click on them or view them too . Plus making sure your sitemap is good. Checking both with www and without in the webmaster tool also.
A "top quality link" will get indexed in a matter of minutes and will also give you a boost in rankings in the same amount of time.
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A "top quality link" got a good one. edmoz etc have taken a lot longer than that even when they finally added the link.
it all depends where its placed, the more recent the cache date and the PR the bigger chance Google will find it.