Patience is a virtue, it's better not do or say anything you might regret later or that might make her feel uncomfortable. What I would do is to continue as nothing had happened (however often you normally text to each other) and have a thorough conversation face-to-face when you actually meet up. (Good communication is essential.)

Perhaps she's had a busy week? For how long have you been together? Have you had the same routines with sms all this time? If the relationship is new she might feel that too frequent texting takes away her space. Are all the texts with similar content? (how are you today or similar) Perhaps it's time to try to pique her interest with something more exotic.

If you're as annoyed with this as I would assume you are, I'd suggest waiting until you see her again to make any moves. Try to have fun with her for a few hours like nothing is wrong, then, as casually as possible, present the issue to her. See if there's anything wrong or if she wants to talk about something. If she acts like she has no idea what you're talking about (hint: she will!), tell her how she's been acting.

Do not do this hostilely. Don't come across like you're accusing her of anything, just that you're concerned and care deeply about her. Remember, the rules of the game differ greatly when you're in a relationship and it's important to show her that you care about her and that, while she is not your only opinion, she is your preferred option.

If she gets defensive, that's a red flag that something is wrong.

Good luck and keep us posted.

Imo this isn't going to be a good relationship. If she takes so long to text/call you that just tells me she doesn't care. Sorry to say man.

Then again maybe its her personality, but why be in a relationship like that? Shes either careless or doesn't like you that much.

Honestly in a relationship you shouldn't have to wait for a girl to text you back. Might as well meet newer and hotter girls who take time to text you back. You'll get the same results.

How often do you see her and talk to her as well? Maybe shes busy and you're going into overkill too. One of you is at fault here lol

UPDATE

Ok so a few texts where sent today and things seemed ok.

At 8pm she asked me what I was up to tonight.

I said I was out with my mate who's visiting from London and asked what she had planned for the evening.

Well I got no reply so it turns out I'm pretty pissed off again...I don't really think there is an excuse for this crap in an exclusive relationship that's meant to be going places!

I'm going to confront the issue tomorrow in person as long as she doesn't flake.

Considering the advice in this thread and others it seems like the best approach is to be my normal fun self for a few hours then bring it up in a non confrontational way. Nearly as if I'm looking out for her...

I'm thinking of saying something like this...

"Is everything ok?
I've been wondering if I've done something to upset you? 
You've been quite distant lately and I feel like you don't want to communicate with me much at the moment. 

I want to make sure your ok because I care about you greatly! I also want you to know that if your not feeling the connection anymore then don't be afraid to tell me!"

How does this sound? Any feedback or advice will be much appreciated!!

I would ask her face to face is everything ok and have I done something wrong? Then be quiet the first one that speaks in negotiation loses always.

The last part sounds weak and needy don't do that, be the strong guy, you don't have to be an a hole but let her know if she says that continues to do that she won't be hearing from you.

The last part is fine, but maybe reword it. I have one tactic that I've used with a huge success rate, but keep in mind this only works if you were pretty decent friends before the relationship.

Tell her something like "Look, I really care about you a lot. You're an amazing person to me and I want to keep you in my life. That's why I want you to know that if you're not feeling like being in a relationship with me anymore, I'm fine with just being friends. It doesn't have to be awkward. We can do it."

You might think it's high risk, but it's not. Deep down, if she just wants to be friends anyway, it's for the best. But otherwise it's like you're threatening her, only without the aggressive part. She'll be forced to get her head together and maintain the relationship if that's what she really wants. No more "on the fence" bullshit. The only thing is, when you say this, you have to truly mean it in every single way. Don't back down.

At 8pm she asked me what I was up to tonight.

I said I was out with my mate who's visiting from London and asked what she had planned for the evening.

Well I got no reply so it turns out I'm pretty pissed off again...