Made my first sale in first days of May, when I had 2 blogs and 3rd in works. Actually only first blog was getting decent traffic, #2 was still waiting for directory approvals.
I have 10 blogs and another 20 splogs or so. 75% of my income comes from 2 blogs
Splogs are a waste of time unless you use them to feed your good blogs. Reason: whole internet is now clogged with crappy splogs and it will be only worse as webmasters continue on cranking up worthless blogs with default WP theme and a couple RSS slapped in.
Actually building a blog is maybe job 10% done, 90% are your marketing skills and how well you work on that blog to appear in SE's. I also think SEO part is a bit more important than marketing.
I'd say learn your niches, be positive and actually talk to your surfer. This way some of your blogs can earn up to $200-300/month (or more). All at the cost of 1 handwritten post per week per blog (15-30 minutes of time per post). This way you can earn a nice income with only a dozen of blogs that you actually have to update at least once a week.
With splogs you still have to SEO them, write good sales texts on sidebars and think about placing ads. For ideas go to blogging forum=>money clinic thread, as well as certain other threads. There's a whole sea of information right there (some might be outdated though).