I'm not too sure if there is a time limit on the length of time. I would say if it is still comfortable then go for it. I leave mine in for hours on some days and minutes on others, comfort is key.

Well I don't think they are inherently dangerous but it does depend on the size and what they'll be doing. Certainly smallish to medium butt plugs can stay in for hours but of course the best advice is that if they start to hurt... take them out.

I know someone who leaves theirs in all day, and they use a big one. He's still walking around so I suspect she'll be safe.

Personally, I find a little discomfort a good thing. I like to resiste the urge to pull whatever I have in my butt out. I don't do it till pain, NEVER. I just fight that feeling my rectum has when it just wants to squeeze all the way closed and not be hindered.

The Medium is safe (and comfortable!) to wear all day long. Just be sure to relubricate every couple/few hours, or when it starts feeling dry. Silicone lubricant would last longer than water-based, so that's highly recommended. As long as she's comfortable and not in any discomfort, it should be fine.