I saw a provider while out of town on biz and once I was hard, she put a cock ring on me that kept me hard for quite a long time (think it keeps the blood up there). Am sure there are ladies here that could provide better insights than my singular experience.  

I know a few guys who use them on regular basis. Generally, you put one on once you are hard. If there is a trouble with erection, a ring is not likely to solve it. If you are a beginner, I would suggest starting of by getting a pack with different sizes. Cock rings that stretch well are the easiest to use. Once you figured out if you like it and what size works you can invest in a fancy one made out of leather.