1. If you are buying links, be sure that it "falls under the radar" because Google is devaluing (essentially making them ineffective for passing page rank) paid links. So, to directly answer your question. . . your websites toolbar page rank may never change or eventually go down, especially if you are buying links.

The "toolbar" page rank is different from Google’s internal page rank.

2. Maybe for a rough gauge as to how many links make up a PR 7 website you should find some & do some research on their incoming links; I’ve never had a PR 6 website & I’ve never worked to increase my page rank. Personally, I would think that only a science/math major would be able to easily work with the equation listed, and there is no guarantee that this equation is what Google uses at this day in time to calculate their internal or toolbar page rank. If you could grab the current formula that Google uses to asses page rank, then possibly you could have a better idea whether or not this was accurate.

It depends on the situation, sometimes, the pagerank updates takes 4 to 5 months after the last update.