Thing 25 is The Blogger's Toolkit.
I added statistics collection with Sitemeter. It's so interesting to be able to see those results.
I added a slide show with Picasa.
I added a poll with a Google gadget.
I added a search box. I think that's a good tool to have on any website.
I read and watched the assigned material. I thought the 20 Usability Tips article was particularly useful, and I put some of the tips to use. Because of that article, I (slightly) more formally chose a topic for my blog and wrote sort of a mission/theme statement at the top. I added a subscription button near the top of my blog. I enabled anonymous comments. I added a photo to my profile (although I'm still leery of making my identity too obvious). I plan to follow some of the other tips as I have more posts to work with.
I decided not to add speech, because it just didn't make sense for my blog. I didn't add a comment tool, partly because I don't want people to rate each of my posts and partly because it made my blog look cluttered. I didn't do the "added functionality" ones because I didn't fully understand what they did and I came up with 4 other widgets I wanted more. It was the same with the social ones, plus I don't want to enable chatting on my blog. I'll probably come back to some of the listed items, but, here we are to question 3...
How much time?
A lot. I took multiple days (5?) to learn, choose, and then set these things up (obviously not whole days, just some hours of each). I kept up with daily blog posts, too, because my blog just doesn't have that much to work with yet, and I'm following the usability tip on posting regularly! I looked into one gadget/feature per day to keep this from taking up all my time. I did sign up with things like SiteMeter, Feedburner, Flickr, & Picasa, so I had that business to deal with for the first time, too. And I tried some things that I didn't keep on my blog.
All in all, I'm glad to know how to add new functions to my blog. I would have been entirely intimidated if I didn't have this good introduction to widgets.
Wait, wait, I just did one more: the Myers-Briggs analyzer for blogs: http://www.typeanalyzer.com/ . My blog is an ESFP (the performers), which is not what I am! But I think I am an INFP; I've got that paper packed in a box somewhere!