What is the best approach to make more money from AdSense – spend your time to one content rich site and maintain it regularly or have multiple sites with little content and occasional update?
I think spending your time on one website, IF it has good keywords. If you have multiple sites with crappy keywords, and one site with high-paying keywords that really makes a big difference in adsense revenue.
Having multiple sites is fine as long as the topics are quite different such that the sites don't compete with each other. Generally, it's more work to try to build traffic to multiple sites and maintain them. If you have one general topic that lends itself to be broken into sub-topics, each sub-topic can be made into a separate keyword-targeted page. So, you end up with only one site, but lots of content pages, each created for a particular keyword, or small set of keywords. I prefer the latter approach because I can focus on getting traffic to the single site, as well to individual sub-topic pages. I've found that once the main site gets indexed and ranked well, any other pages that I add are also quickly indexed and ranked. This doesn't work as well with the multiple-site approach. I also cross-link pages to get maximum SE and visitor exposure, which is also more difficult to do with mulitple sites.
I agree, try to focus on multiple pages with different content on one site. That would be the easiest to maintain, and create the most revenue.
IMO Approach is different in all niches. You have to analyse your visitors behavior on your site and then putting right adblocks with right blending works great.
I've always advocated small niche sites that do offer substantial information to the visitor. Whether it's in blog form, an occasional posted article, or even a combination of rss, blog and article- give them something to read.