Blood clots may be a concern. When I did a quick search around, it seemed like the general recommendation from most places that sell them was to remove them every 20 minutes (or sooner if you experience discomfort).
This is the general warning that you’ll find at most places that sell them:
“Warning: do not use cock rings if you bleed easily or have a blood clotting disorder, if you are diabetic, or if you suffer from any peripheral vascular or nerve disease. Do not use these toys if you are taking anticoagulants, aspirin, or any other blood-thinning medication. In the event use causes discomfort or any unusual body changes — including bruising, pain, bleeding or loss of sensation — discontinue use immediately and check with a physician. Do not place over sore, swollen, or infected areas or areas without sensation”.