All the 'stretchy type' of cock rings can only be used for a short period of time, usually 30 minutes or less. When the diameter is anything smaller than 1 3/4 inches, this will restrict blood flow.

But metal cock rings, like stainless steel, are wonderful especially when there is some weight involved. Once you have found the correct size, you can wear the cock ring for much longer. I have worn a 2 inch cock ring for several hours with comfort. The metal ones have some attractive designs too that add to their beauty. But again, determining the correct size is critical. Most of the metal cock rings come in a range from 1 3/4 inches to 2 1/4 inches in diameter (inside diameter). The average size falls somewhere in the two inch diameter.

To determine the diameter that feels right for you is to take a flexable tape measure and wrap it around and behind your cock and balls and draw it closed to the point where you feel the most comfortable. Find the length on the tape, this may be say (as an example) 6 1/4 inches. Using a calculator, take the number 6.25 (6 1/4) and divide by 3.14 (pi) to get an answer something like 1.9904459 inches or, in this case 2 inches (diameter).

I guess I exceeded the scope of your question, but I do think a metal cock ring is far more gratifying and attractive in the long run.