The person who speaks last isn't always the person with the most power.
Basic rules to keep her attracted:
1) End the conversation first. This is superficial, and it works. Just don't be rude or obvious.
2) Don't respond to every text message instantly like you're a lost puppy dog just waiting by the phone for each and every text. Finish what ever you're doing first, and THEN get to your phone.
3) When it's getting late, and you don't want to leave her hanging, make sure to make your last text obviously the last one like, "haha, you're so right! Any way I'm hitting the sack now, have a good night! x"
How do I know doing these things help? Because some of this is exactly how the most beautiful girls I've ever dated would text me.
It really doesn't matter. If you have things to do, just explain you are busy at the moment and will text her later. If it's getting late and you're tired, just say you're going to bed and ask them if it's okay if you text till you fall asleep because you don't want to go just yet. And I would reply to a guy any time before I go to bed on school nights usually around at 10:30 but on the weekends it doesn't matter.