How many days for new backlinks to influence Ranks?

It depends what keyword you are trying to rank... But for estabilish your ranking i think 3 weeks is good time.

It really does depend on the keyword you are trying to rank, I have known it to take anything from a few weeks to longer to rank. 7 - 10 days used to be the norm but I do believe they have increased that now so 2 - 3 weeks sounds more average now.

The newly placed links will not influence the position of your site until they're indexed.

If googles bots rarely hit the sites the links are placed on, then it could be some time before you see them influence the site they're directed at.

When you place a link, you have to get google to visit the page to see/index the link.

I've heard tweeting the url of the page that the new link is placed on can speed it up. I've heard posting a link to the page on facebook or other social media can have the same effect... What's important to speed the process up is to get google to somehow see and index the link.

Or you can do like me and just be patient and let it happen naturally.

How long does it take? That's like asking how long is a piece of string.

How long it takes to influence your sites position in the SERPS after a back link is placed has nothing to do with the keywords your after.

How far that the placed links will move your site in the SERPS. Or if they move your site at all is determined by the competition for the key phrase/word you're targeting, and the value Google places on the links you've placed.

When a typical site say ranks somewhere around the 100th position for it's primary key phrase, then moving it to the 50th position with back links is fairly easy. Obviously not that many of the sites ranking down that low are being done by people that know what they're doing.

And moving that same site from the 50th position to the 20th spot will be harder and take more link juice to move than moving from 100th to the 50th position.

On the same note, moving the site from the 20th position to top 10 and even the top 3 positions can be a beast if it's a competitive keyword and will take a lot more juice than it did to move it from 50th to 20th position.

Like I said though. How long a backlink takes to affect a site has nothing to do with key word difficulty. Keyword difficulty has to do with how far it moves or if your efforts were even enough to move it at all depending on what position the site is in at the time of link placement and degree of competition for the ranking spot you're going for.

Yes... the rank will increase for sure. But it won't last long. Search engines are smart enough and they can see that you are getting link from quality site or just ordinary sites which are providing links too often.

There is a debate about new sites getting sandboxed and not entering into the top page of Google for the first month (not sure if yours is a new site).

Also, it's not about the PBN, but the quality of the page that it is linking from. The strength of the PBN should be shifting the site really quickly and 6 months is very strange. I can only point fingers to the quality of the site; I have managed exceptionally well on a small number of sites on the PBN, web 2.0 sites and directory links to get a site on the 2nd page of Google for 10 keywords. The client did not have the budget so had to improvise.