How long until Onpage optimization has some effect?

I have been optimizing some single posts and tags on some of my sites and I noticed that it can take up to 1 month for it to jump a few pages. Is that the usual time?

1 month is a long stretch. From my experience, anywhere from a couple days to a week. Build quality links to your site while you wait.
For example, my money site was on page 5 for my main keyword, and then I figured I wanted to rank for my new secondary keyword which I never placed in my title tag and content. So I altered the title tag and content of my site to include this keyword and when G re-crawled my site, it dropped down to page 7. I just said whatever and kept building my quality links. That was about 3 days ago. From this morning, it's sitting at page 4. Mind you it's a very tough niche too. So yeah, just keep building great links and relax.

It varies. I've had new blog posts turn up at #1 on Google for my chosen keyword within an hour of posting. Not often, but sometimes you get lucky.

If it's taken a month, I'd say you need to get links or build out the content a bit more. I never create blog posts of less than 1000 words.

Charlie

From Minutes to 48 hours

Google hits sites with quality content more often. It depends how much google likes your site, if its going back and crawling your site more often then you're going to get indexed faster.

Pump out quality content more frequently and google will start indexing new pages and page changes really fast.

It depends on how you've done the rest of your SEO work. If you mess up the inbound stuff, then no amount of on-page work will fix it.