Size Doctor session length

I’ve seen plenty of recommendations for session length for conventional noose/strap extenders, but what is the recommendation for the Size Doctor style vacuum extender? The one I bought turned out to be counterfeit, and the instructions are in (presumably…) Chinese, which doesn’t help me at all…

When I wear it I haven’t noticed any circulation problems, though my glans gets pulled out of shape a bit by the vacuum. But before I try any extended sessions, I’d like to know if there’s any reason not to.