However long it takes for Google to find them and then process them. 

Google crawls different websites at different rates. This website is crawled constantly. New pages show up in Google's index in seconds. So links found on this page are available to Google's algorithm almost immediately. Pages on a personal website that has few links and little content will be crawled infrequently so links on those pages will not be found by Google for weeks or months. 

Google supposedly is constantly running its ranking algorithm so theoretically as links are found they should have an effect on a web page's rankings. But who really knows how often ranking calculations are performed? Plus who knows when data is actually factored into a rankings calculation? It may vary like crawl rate does. 

So the answer is: it depends and varies a lot.

How long does it take for backlinks to appear?
Is there a way to check which server OS a particular site is running. For example from linux command line. Maybe via telnet? But all I can get from "telnet www.google.com http" is:

Trying 209.85.173.104...
Connected to www.l.google.com.
Escape character is '^]'.


Can somebody dwell on the subject if possible at all?
