It is very INTP to forget to reply to things. I get very caught up in things, too, to the neglect of other aspects of my life. If it's normal behaviour for her, I expect you have nothing to worry about.Yea I am pretty sure it is circumstantial, especially since this is kinda normal to not hear from her for a while, and I just need to understand that; she's at school, she works, etc. Plus we don't text, we talk over facebook. Often enough I don't see her online, or message seen, for a week at a time.
She also gets very caught up in the things she is doing which I've heard is an INTP attribute(?). For example once she hadn't replied for 3 weeks than I sent her a little message, or something, and she got back to me like nothing happened and felt bad for having forgotten to reply. Still don't see how someone wouldn't have the time to respond with even a short message "I want to but I am to busy so I can't". Then again she doesn't really message like that and I've never said that I appreciate even that type of response.
I don't like to message multiple times in a row so if she never replied it leaves me hanging as to when I might talk to her again. Although is it an annoyance to get multiple message in a row? I don't necessarily find it annoying but I know if your busy and caught up in something it could be?
If I were you, I'd talk to her about it. She probably has no idea that other personalities want more communication, more often - to us not talking to someone for a while doesn't mean we're not thinking about them, and I think we don't realise other people don't see it the same way. For me, it really was something I had to consciously learn - that I should talk to people regularly in order to maintain relations - and I still have some trouble with it now. I'm having trouble thinking of how you could phrase it, but just tell her that you'd like it if she could shoot you a message a bit more often... and you get she's busy, but you'd like it if she could tell you when she's unavailable rather than leaving plans hanging in the air. It probably hasn't occurred to her. If you get the tone right she probably won't take offense. I mean, it's a perfectly reasonable thing to ask.
As for multiple messages - in terms of the awful games that run through dating, it's kind of a faux pas to send two in a row... not so much because it's annoying to receive, but because it gives the impression of there being more interest on one side than the other. "She who texts first loses", and all that. But in this case, it sounds like she's forgetful - and a second notification will jog her memory. I think that if you're trying to make plans and need a clear yes/no response and aren't getting one, it's totally fine to send multiple messages.