You would be better off posting this query in the Google forum.
My two pen'th:
1. They will do it when they feel like it
2. PR is just one factor in determining how your site will perform for any given search arguements. Poor PR does not stop your site doing well in searches if you have good content.
I don't think there is a definitive answer from Google. I've heard everything from 4 weeks to 4 months.
It has been sugested, by google chaps , that googles internal page rank computation is continous, so keep building links, google knows about them quickly.
Toolbar page rank, what you see on google toolbar is exported by google to the toolbar, perhaps 4 times a year,
Try an forget about your visible toolbar PR, its a waste of your time worrying about it
Focus on everything else an the visible PR may catch up some day
But if you're top of the SERPs by then, you won't care too much :)
It could take over a year!
I haven't checked PR for at least 6 months. The site comes top 3 in most of our niche search arguements, that is what matters to me.
I have had some success getting other related sites to link to my site but I think it would be easier to get sites to link to me if I had a page rank higher than 0.
Thanks for all your help so far,
yetoyeto, it's very difficult to get sites with PR to link to a site with 0 or no PR.
Try finding as many directories as possible. Also trade links with other sites, no matter what their PR is. In the beginning, quantity of links sometimes has to be a substitute for high PR links.
Once you get your PR, you'll have other webmasters bugging you for link trades.
In the meantime, keep at least some of your focus on those "related sites". Page rank isn't the only benefit you can offer them, and links related to your site content are likely to help you more with google than are unrelated links.