I was told that 16 hours is not really long enough.
My understanding (not saying this is accurate, just that it is my understanding) is that the body starts going into "fasting mode" after 12 hours or so, the response to fasting peaks at about 16 hours, and it more or less keeps up until you eat. Assuming that's true (let's all agree, the science is far from definitive), then a 16 hour fast most days would yield substantial benefits. So would a 24 hour fast a couple times a week.
Personally, when I train in the morning I eat three meals a day, and when I train in the afternoon I fast/under eat until shortly before or after I train. This means a fast/under eating period of anywhere between 14-20 hours, 3-5 days per week. Some days I get hungry after 14 hours (although that hasn't happened for a while), and other days are great. Today was actually exceptional. I was busy all day, I commuted on my bike, and was able to be very productive and focused, all on about 16 oz of coffee and a handful of pistachios, spread throughout the day. I got home, did swings and clean & presses for an hour, still felt fine, and ate about 20 minutes later. It ended up being 22 hours between meals, and I'll have my actual dinner here shortly (about 2 hours between meals), and that should do it.
I was mostly worried about losing weight (read, muscle), but I have neither lost nor gained weight. I probably eat fewer calories on the days I fast, even though I eat as much as I want to in the evening, but it seems to work out (some people say your body utilizes protein and carbohydrates more efficiently after a fast, I'm not sure I buy it or not). I just love the fact that I don't have to be married to my food, or worried that I'll shrivel up if I don't eat enough. And things get interesting after your 2nd or 3rd cup of coffee on an empty stomach... I think the key is to realize that your body has to take time develop the right cellular machinery before you ask too much of it. Feeling "different" is not the same as feeling bad, but if you do feel bad, pushing it will rarely have good results.