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.
When i started out using butt plugs i did it for a hour a day and each week i eateneded it a hour each week. Now im so use to it i keep it in all the time. Icant be with out it