Generally for me I wait at least one "turn" of the PageRank and possibly two. Closely monitor when Google does a PageRank update and check your PR then to see if what you wanted occurred.

Google recalculates most things that matter in close to real time these days. Toolbar PageRank is not something which matters. Updating toolbar PageRank on a once-in-several-months schedule is largely for historical reasons, does nothing for your website, and is kept around virtually entirely to placate people who know just enough about SEO to hurt themselves.

Toolbar PageRank updates are done on a rolling basis too now, they stopped doing massive updates every few months some time ago.

Patrick is right, PageRank is basically a completely worthless indicator. Monitoring your position in the SERPS for lots of keywords(including long tails) is the best way to see if SEO is paying off quickly.

Any recommended tools for monitoring keyword positions?

There was a startup posted on HN recently that did this, I can't recall the name.

Honestly, it's super easy to roll your own and run it yourself. That way you're not giving up (potentially very valuable) keywords to anyone in the cloud.

Insider Tip: If you're only interested in the top 10 results, use the Ajax API to get around query limits.

Ok, so I've got ~15K pages on my site now (it's a directory for mobile notary publics) - how long would you wait in between page-level tweaks?

(I notice that Google crawls about 500 pages per day... So, should I be waiting at least 30 days in between tweaks to let them crawl everything?)

I don't see any reason why you would wait at all for a page level tweak which you believed was likely to be beneficial. (Well, I do, but it isn't relevant to your situation.)

You're not obsessively tweaking the same elements over and over again, right? That would not be strategically wise. Instead, make a best guess now, then move onto strategic issues like "How do I get links to my website? How do I get links to my website at scale?"

In general, it is very, very, very difficult to map specific on-page micro-optimizations to business-level successes. (There are times when this is not the case, but a) you'll generally have had a older website b) you were doing something extremely wrong, like not having title tags for every item in a 200k page catalog and c) you will see almost immediate, dramatic results.)

My experience with "What happens when PageRank rises/falls" does not mirror what you said and I've been doing this for 4-5 years at a "basic" level. When a PR change occurs on our site, we definitely see traffic shifts both in terms of the number of visitors and in the quality of traffic. No question. The most recent one I remember having a big impact on us was May, 2010. We had about 3 pages go from a 4 to a 0 (still don't know why) and almost immediately our traffic numbers + quality for those pages was #$&%.

Note that the change in traffic was right at/around the time of the PR change - not a month before, two months before, or even a week before.

So, from what I understand, it's a period of months... rather than weeks or days? If so, what do you do in the meantime?

Well, I work on other aspects of the business. Some things I know whether they work because I can see them in the serps. Others - like trying to increase PR - take longer.

SEO isn't my #1 priority right now so I'm not really focused too much on it ATM. Right now, I'm heavily getting content together. I've only done a few things here and there for SEO in the past 3-4 months.

if anyone is waiting for the visible toolbar PageRank to increase or decrease, i seriously doubt their high level knowledge of SEO. Toolbar PageRank is simply a historical indicator. Many times it is weeks, if not months old. And if you do any kind of research, you'll see that PageRank does NOT equal rankings. It can help as a strength indicator of your site, but correlation does NOT equal causation.