There are tons of vids on youtube, you should definitely research it if you are into health, Dr. Herbert Shelton was a pioneer . How tired you get depends alot on how destructive you were to your body but generally the longer you fast the weaker you feel. At this point I have done a 23 day water fast, and a 31 day water fast- for me the first two weeks went pretty easily and I had a decent amount of energy (not out doing vigorous activity though). To me you want to do at least 10 days or so because it takes about 3 days just to get your body into FULL FAT BURNING mode where pretty much all you burn is fat. Also it takes some time to recover, the shorter the fast the easier. My 31 day fast where I got sick at the end (I did not do an enema to clear my intestines that were blocked (otherwise I'd have been fine ) as soon as I did I felt better) took me a month to fully feel better. I used to have stomach pin EVERY DAY, I am now feeling better than I have in 10 years.
This guy I like:

and this guy (I don't think you need supervision though, just listen to your body) PEACE: