I have been wearing cock rings for years and have quite a collection. I started out with a 10 speed leather snap ring. It was great and I started wearing it regularly. I moved up to a stainless steel ring and WOW, what a differance. raging erection, looks incredibly sexy, and a lot more comfortable to wear on a regular basis. I have three differant style steel rings, two leather snap rings. I also have a leather snap ring with two rubber rings on the shaft. that one is really hot looking too. I got the steel tear drop or tounge cock ring about a year ago and it is by far the most comfortable ring I have ever worn. I just ordered the most unique looking ring I have ever seen. It hasn't arived yet but it looks soooooooo hot. The web site promises that it is extermely comfortable too. If you are into cock rings you have to check this one out. It's called The Spine. It's on the front page of the web site at Welcome to Mr.-S-Leather.com
Forget the math to figure out your size. The girth of your cock is not the only consideration you need for a good fit. After all, your scrotum goes through it too. I would try a leather snap style ring to find the fit you like and then just measure the ring at the adjustment you like. It also depends on whether you are shaved or not too. Also, the tear drop or tounge ring fits a little tighter than regular rings of the same diameter. I wear a 1-7/8" ring, but the 1-7/8" tear drop or tounge ring is slightly tighter than my other 1-7/8" rings.
As far as how long you can wear them, there are a lot of web sites that say 20 minutes max. I think they are talking about how long you should wear them with an erection. I often wear them for days at a time. The key is that it is comfortable and not to tight that it would cut off the circulation to your cock. One web site I read said that if your genitales feel cold, it may be to tight or you are wearing it to long. That sounds like good advice to me, but, just remember that your testicals are designed to be cold. That's why they are on the outside of your body.
Contact me if you want to talk more about cock rings. I am a real advocate.