@feralmoan,
I think this is one of those cases were its sort of hard to compare aggregates since your return on investment is personal to you. In some cases my blog posts have literally taking man weeks to create because they are based on efforts for customers. The code, data, and experience end up getting written down then at some point I might spend 10 minutes to a few hours writing a blog post. Some of those very same blog posts, that have consumed massive amounts of time, have been more or less ignored. Some have ended up being very popular. So what I would say is ignore your average investment time and focus on your goals. What are you goals for blogging? Is it just expressing yourself, communicating to others, sharing your hard won experience, driving a brand (maybe brand you), or just storing information for latter retrieval. Depending on what your goal you will find that you time investments will be very different. For example if you’re really want to show the world how deep your understanding of certain technologies are you might need to spend significant time writing rich and deep content. If your just sharing news articles of interest with a brief bit of insight maybe just 10 minutes a day.