Page ranks has nothing to do with good quality traffic and content. PR is based on backlinks. Now the theory goes if you generate good content you'll get those backlinks. But you can have an absolute crap site with no traffic and still have high PR.
Also if your site is just a few months old your website will not show any PR in the toolbar. I have a site that gets about 150-200 visits per day and its ranks well but it has no PR.
Google PR do matter. It is something that can't be ignored on any cost. We cannot give exact timing that when will Google update PR of sites. But usually Google update the PR of sites after three months. But this is not a hard and fast rule. It depends upon many things including the number of quality back links of your site.
Thanks.
Since Google ranks your pages according to the number of links pointing at your page, it stands to reason that you should try to get as many links pointing at your pages as possible.
Although the PR matters to some extent, in my opinion, you should not worry so much about pagerank if you have good rankings for the keywords you target.
PR is just one of the ingredients in the recipe of achieving good search engine rankings so, if you have such good rankings already, don't worry aboyt the pagerank.