1. ideally, yes. But even if the entire PBN site is not topically relevant to your money site, you can still reap the benefits of the PBN links if you put those links inside of content that's at least remotely relevant to the content of the destination page on your money site.
So, for example, let's say that your money site is centered around obesity surgeries and one of the sites in your PBN is about cutting edge technologies. Well, as long as you can post an article on your cutting edge technologies site that is somewhat relevant to the topic of obesity surgeries, you will still get a great backlink to your site. And one way of making this relevancy connection that you can do, and which I can think of is by creating an article on cutting edge laser machines that are used to treat obesity. Such an article, published on your cutting edge technologies website is relevant enough for your PBN site (because it touches upon the technology aspect of the PBN site), and it's good enough as a backlink to your obesity surgery site because the article talks about laser machines used to treat obesity.
2. ideally, an unlimited number because the best PBN sites are the sites that look, feel, and behave exactly like money sites because, in reality, top notch PBN sites ARE money sites. So, if you can create 100s of money sites, all of them ranking and making you money, then you will have the perfect PBN network that google will hate you for
So, the number of articles that you should submit to all of the sites in your PBN network should always be unlimited. But if that's not an option (because you're lazy, bored, poor and can't afford churning out content like a zombie, or any combination of these 3), you can just publish as many articles as are needed to keep those sites ranking in google and getting you traffic, because a PBN site that has no traffic is a bad PBN site and the backlinks coming from it are usually low quality, as well.
And once the PBN sites start ranking and getting constant traffic each month (even if it's only a few 100s visitors a month) just make a habit out of publishing few articles a month to keep the sites updated so that google doesn't de-rank them on grounds of being dead in the water
3. the duration doesn't matter. What matters is that the link comes from indexed content. Although, some folks claim that this is rubbish, but I prefer only getting links from indexed content, so to each their own I guess...
