If I get a link today when will it start to affect my google rankings? Is that called a algorithm change?
You have got it wrong. PR and BACKLINKS are two different things. In reference to your question - your link can take up to 3 weeks to show up in Google Webmaster Tools. However, Google is able to detect link activity on a daily basis.
After indexing the backlinks you will start to get the link juice. And the indexing in search engine depends on the nature of the link. If your backlinks are DoFollow and are from high PR popular pages then they will be indexed very fast.
I think there are 2 possible cases here. On the first one Google would update the SERPs in real time and constantly, as soon as new information is retrieved from its crawling bots. For example, if a bot was crawling a certain page and discovered that the Title tag was changed, it would immediately report it back to a central data center, that would in turn update the database and the ranking positions for this particular website.
The link you get back to your website will be in effect after crawl by the Google bot. It may take 1 -2 days.
Well, I think the simple answer would be often. But, if I think more and more, I think there are 2 possible cases here. On the first one, Google would update the SERPs in real time and constantly, as soon as new information is retrieved from its crawling bots. For example, if a bot was crawling a certain page and discovered that the Title tag was changed, it would immediately report it back to a central data center, that would in turn update the database and the ranking positions for this particular website. Second, Google has a fixed schedule to update all the results together. The schedule here could be daily, every 48 hours, every week and so on. For example, it could do it once every 48 hours, and on every update it would combine all the data retrieved from crawling bots and recalculate all the SERPs simultaneously.
Very often, like twice a month. But this update is not visible in Google bar so often, in Google bar this update is updated 3-4 times a year