I'd recommend adsense as a last resort. If you truly want to make something of your blogs link them to each other and build yourself a good traffic between the blog network. Use one blog as ad on another. Thereby building a fantastic flow of traffic and retaining visitors within your network. This will compliment all your sites
Remember the most ignored downfall of ads on a site: They lead the visitors OUT/Exit not bring them in. Unless ads can offer the visitors something relatively useful to what your site and their interests are then they are just ugly or loss of visitors.
It would be much better to use affiliate marketing links/ads which are related to your site content.
Placing ads is advisable when you have a regular traffic coming in month by month but the affiliate conversion are not getting higher.
There is no specific number but I have found that if you are getting over 100K impressions a month for an ad zone you can get placements which bring in higher revenues than bulk/network ads.
Source: Personal 8 years of experience plus advertiser's perspective.