This is one of the top questions I get when we start a client out with a new blog. My answer is always the same, as long as you write with a purpose and bring across your point clearly your post can be as long or as brief as you want it to be.
This of course depends on the type of blog that you manage. Is it tied to your business? If so, I recommend that you maintain a professional blog and keep the personal stories a minimum in which case all of your posts should have some kind of point to it. Thus, each blog post should have a few good, poignant paragraphs – and generally, don’t stress about it.
A few tips: go back to what you learned way, way back when. Hear that voice telling you, “Have a solid introduction, a body and a conclusion.” That gives you about three paragraphs right there. Make sure to include some links as references (if for instance a source is not well known, you will want to link to that site). You can also include an image or two (with the appropriate Alt Tag too of course). And voila, you are done!
Hope this helps take a little bit of pressure off your weekly to do list. Oh and look, three paragraphs and I made my point.