If i got your question right, you wanted to know if you get a backlink from A website, how much time will google take to scan that backlink which is on A site? Well, if thats your question then it completely depends on the popularity of A site. For example, if you post a link on DP, it should be scanned anywhere between 1 to 6 minutes.
google always shows a little backlink even for a big sites. I have seen that most websites which are very old and have thousands of backlink, their backlinks are shown in google only 3 or 4 pages. Don't worry about it.