Her room has a short hallway past the door, so you have to open the door and walk in a step or two before you see the other half of the room and the bed.
How often do you think I find girls in that situation? I probably wouldn't do it like that with anyone else unless I thought she was into me, and that's really the only way I'd be so brazen as to just walk in unannounced. I only did it with her because I felt fairly certain that she'd enjoy it. I have tried to do it again a few times that I found her like that, but it didn't really feel the same as it did that first time. Keep in mind, I only moved to her town about 2 years ago, and this was closer to the first month or so of my move. Her sex drive is usually pretty high, and she masturbates a lot because she doesn't really like dating, but she also doesn't want to have the reputation of 'slut' which means she's often very frustrated. That being said, although she let me keep the key and actually encourages me to walk in unannounced (we're very comfortable with each other), I don't go over there specifically to see if she's masturbating again. I'll occasionally make a social visit, and I'll pop my head in and check, but she's often just playing games or watching TV, so no, it's not something I do very often. When we do hook up, it's usually because one of us calls the other and specifically wants to hook up, but we did take a break for a few months because she was seeing someone up until a few days ago. Before the break though, it was something like 3-5 times a week. More than I would get, normally, but far less often than I would like, ideally.
I also cannot in good conscience recommend it unless you're already very close. I don't know either of you, but use your best judgement. If she resists at any point, stop and think about the consequences for both you and her.