It usually depends on how much tension you apply, really. I found myself able to go for an hour. As for the plug sitch: vaseline is a great idea before putting on the device. But it still doesn't solve the problem completely. Perhaps sili putty or a small rubber stopper like that found on the...
I just purchased the heavy duty staticstretcher and the biggest pain is getting a good seal with that dinky little rubber seal. Other than that I get a pretty decent stretch. They need to use a plug like Mynewsize does on their MV extender to get a decent seal in the vac part.