It depends on how long it takes for Googlebot to find the new link. This could be anywhere from one day to two weeks, depending on how often the other site is crawled. After the new link is found, there could be an additional delay of 1-3 days for it to be added to Google's database. There could also be an "aging factor" on the link, so that it won't give its full value for several months.
Mass bookmarking sites like socialposter.com or socialmarker.com can speed up the bots. I got it within a month.