wondered if Google for example has an 'age factor' - ie a length of time before it will take into account a link or whether once crawled it is 'counted' straight away....
What a seemingly simple question that generates a very short and long answer at the same time... ShortVersion :
Short Version of the Long Version:
It depends what type of impact you're asking about. PageRank:
The link likely goes to the back of the queue and gets processed on the next pass -- Takes a while. TrustRank:
(Based on "hops" to get from a "trusted site" to the linked site) Likely similar to the time it takes to have PageRank applied -- Takes a while. Relative Freshness:
Likely nearly immediate (no point in trying to figure out what was "minty fresh" 3 months ago to show people today), but also likely on a per-niche basis, and only likely to have a "visible ranking impact" on QDF queries and the level of impact would also be dependent on the "freshness" of the page it's on, and the pages linking to the page of site it's on, and where it's place and how likely it is to be clicked and a quite possibly a number of other factors I'm not thinking of right now probably play a role. Relative Link Churn:
It's fundamentally "backward looking", but the link would likely be "counted" relatively quickly, while any impact may not be felt soon if at all, since it's probably a calculation that's "averaged over a period of time", rather than a "number today" type calculation. Relative Growth Rate:
Likely counted nearly immediately since it gives an indication of a "hot topic/site" right now compared to other sites in the same niche, but any positive effect of it for "hotter than the rest" would also be short-lived, since it's calculation that would look for "more popular today" rather than "overall totals".
There are probably a few more I'm forgetting, and we can't even be sure they use all of those specifically, but they are all from Google patents, so I think it's reasonable to assume even if they don't use what they outlined/patented specifically they likely use similar concepts for document scoring based on inbound links, and the overall impact of links or any given link isn't quite as simple as PageRank passed and the answer to "how long does it take for the effect of a new link to be felt" is really, most likely, it depends...