I'm Amanda Olson from IntimateRose.com, and I'm a physical therapist specializing in pelvic floor rehabilitation.
One of the questions that we commonly hear at IntimateRose.com is, “Can I use the weight longer than 15 minutes?”
You certainly can; you're not going to do any harm. I wouldn't go much longer than 18 minutes, or 20 minutes, with the weight in.
If you're able to easily maintain it for that amount of time, and you've gone to IntimateRose.com and checked out some of the more challenging exercises, it's time to move up to a heavier weight for more of a challenge, so that you can continue to get stronger.
You may find that when you move up to the next weight you're not able to maintain it for the full 15 minutes, so just do your best and hold it as long as you can.
It's still better to be progressing and moving in the direction of more strength rather than plateauing and going longer than 20 minutes with the same weight in.
For more educational videos, you can visit Intimate Rose on YouTube or at IntimateRose.com