I use kidblog.org. It allows the students just to view each others blogs. You can choose to monitor comments to make sure they are appriopriate.
I teach computer class for grades 1-12. I use blogging basically as a beginning class activity (for mid and high), so I can take attendance and take care of other beginning of class things that I might need to.
For the middle school grades I usually give anywhere from 5-15min depending on the topic and if it pertains to class. I often choose the topic, so then it would relate to what we are doing in class or school, but I sometimes have an open topic and they can write about anything they want. If they finish they can comment on others' posts.
I have had the 3rd-5th graders blog as well, the 3rd graders are getting introduced to it and need more structure of what to do and what to type. The fourth graders basically want to do it as fast as possible. -Who ever gets done first wins something apparently but I have yet to find out what- The fifth graders are awesome at blogging. They have inserted pics that they have made and comment on others. They tend to take longer to finish their blogs, but it varies.
For the younger grades I usually give them a topic and we brainstorm what they might want to write about. Sometimes students in the older grades will say they don't know what to write about so then I will ask the class for some ideas to get their minds flowing.
Hope this helps here is a link to the 8th grade blog
http://kidblog.org/LCA8th2/ - you do have to be a member to view them -
and the 5th grade blog
http://kidblog.org/LCA5th/ - they have not blogged for a while because of other projects.
I hope this helps.