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If you mean when will it show on the toolbar, it could be months. I have pages linked from a PR6 that were added on 8/1 and now 2 1/2 months later are still not showing any PR on the toolbar.
If you mean when will it have PR for search results (though not showing in toolbar), I believe it is about 2 weeks on the average. At least, that has been my expeience with pages linked from a PR6 page. I believe it is an 'estimated' PR. However, that 'estimated' PR does not seem to be passed on to other pages that it links to. It only seems to start passing PR after the PR shows on the toolbar.
If you mean when will it have PR for search results (though not showing in toolbar), I believe it is about 2 weeks on the average.
(this results in increase in what you get doing link:www.mysite.com search)
No, backlinks get updated at about the time TPR is updated, and even then you can't really rely on them. You won't see them for new links for quite some time (if at all).
Only in 2 weeks these changes "come in force"
This is highly dependent on how often the linking pages are crawled, that is PR and modification stamps.