In addition to my own experience, Guy Kawasaki also said that his blog posts took him about 8 hours to write on average. He's sort of the archetype because originally every single one of his posts was approaching the platonic ideal, and he made the technorati top 100 only a few months after starting his blog.
In general though it's not that hard to tell roughly how long a blog post took to write just by reading it. The typical breakdown of a successful blog post is usually something like:
1) Two or three hours of reading a few journal articles or digging up a bunch of facts and citations from Google.
2) Two or three hours writing down what you want to say in roughly the order you want to say it. The sweet spot is usually 800 to 1200 words.
3) Another two hours restructuring the writing so that your the voice, flow, formatting, grammar, etc. maximally resonate with the people who you ultimately want to upvote your post.
4) Another hour or two polishing the post, especially the first two or three paragraphs so that you can maximize the amount of people who actually read what you write once they click on the link. Also optimizing the title, coming up with a snappy conclusion, making sure the whitespace is aesthetically pleasing, etc.
5) Another hour to turn the post into HTML, make sure all the links are working, fix HTML formatting issues, correct last minute typos, and go live.
If you spend longer than this doing research and coming up with more original ideas then your posts will no longer resonate with as many people because even though it's more insightful, it's less anchored to stuff they already know and accept as true. And the only people who can really do a good job faster than this are those who can get away with writing in the same voice they speak in, which is very few people because most people aren't very good at speaking. So generally only people like Seth Godin can do this and still get lots of upvotes. Similarly, you can also shave off some time if you're someone who can draw on vast amounts of professional experience in addition to research, like Fred Wilson. But again there aren't that many people that can do this and still be credible, because even if you're legitimately a well-recognized expert it still only works for people in a handful of professions.
In general you will rarely see Paul Kedrosky's blog making the front page here, because most of his blog posts are just a list of new statistics without much explanation. And even though these are often brilliant and have the potential to completely reshape your worldview, people apparently have trouble understanding the implications without any further explanation. And similarly, rarely do Salon.com or Edge.org articles make the front page; people have trouble believing that Salon articles are true because there's a lot of research they haven't heard before so they think that it must be false because if it was true they would have already heard about it, and in the case of Edge.org people don't like to think for themselves about whether or not the vast amounts of original ideas are likely true.