If I come upon a new blog, I try to read the entire thing. If the blog has been around for a while, I go back to the first post, read some of the recent posts, and maybe check out some of the posts in between. If I find the blog particularly interesting, I might read more, depending on how much time I have.
I try to comment on at least five blogs per session. Because there are blogs I cannot comment on (foreign language, commenting limited to members only, etc.), I generally view more than five blogs per session.