This article is from the Impotence FAQ, by Sine Nomine llama@drizzle.com with numerous contributions by others.
The manufacturers recommend leaving the ring on no more than 30
minutes, but posters to the group have reported leaving rings on for
much longer with no problems. Osbon, a major pump manufacturer, says
they have no reports of serious side effects from pump use (losing
your penis would be a serious side effect). The rule here seems to be:
don't be stupid. If you notice a loss of sensation in your penis, take
the ring off immediately.
 
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