I can wear a steel ring for a few hours before it starts feeling uncomfortable. I can wear silicon ones all day, though personally I would advise against wearing them while you sleep, just in case something goes wrong.
In my experience, wearing rings won't initiate a boner, but they will make one last longer, especially if you wear a tight shaft ring, as it restricts the blood flow. Rings can also give you a firmer erection.
You can wear a variety of rings. I have smaller ones that fit around the glans, some for the base of the shaft, some that fit the ball sack, and some larger ones that fit over the whole package.
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