I guess it takes a lot of time. Change a thing, wait for Google to crawl your site, then inspect the results and repeat. Most of it is waiting for Google. So yes, I suppose you can do it on the side.

How long is that cycle for sites that Google crawls daily? Can you determine the results of a change right after the crawl, or is there lag time?

The google webmaster tools give you the detailed search results for your site on a daily basis. I'd say you can make week to week comparison at best.

So you just read about seo from time to time, get a few ideas and select one that you think will fit your circumstances, try it , and wait ?

I would say, you create an interest in SEO/SEM - become aware of the various types of things you can do, then start experimenting with incremental changes to the site you're trying to improve. Some changes takes hours, some weeks, the results - similarly - will show in weeks or months. (i.e. getting back-links is extremely tough and time consuming and the results aren't immediate, but once you're in..you have a tremendous boost in rank improvement)