Depends entirely on the post - I've posted two thousand word blasts which took a couple of hours to write (a review written as I was watching the film), compared to a post about the chronology of Alan Class publications which I have been researching since... October last year? Earlier? I still need to fill in some of the missing dates, but it is probably as close as it is ever going to get, unless someone pops up with an account of when each title was sent to the printer. In total, several hundred hours of work has gone into it. There are no rules - you decide how much time, money and energy you are willing to invest into each post. You set the agenda, and hope that people appreciate the work you are putting into the blog.