If she takes, say, half an hour to reply back to your initial text, and you reply within a minute or two of her response, you come off as very needy. (You also come off as a bit creepy, like you were just staring at your phone like some kind of Fatal Attraction-type character, just waiting for her to answer you, and when she did, you answered her right away.)
Another quick point about keeping her interested has to do with texting her after you’ve set up a time to hang out together. Some guys (and I did this a lot when I first started getting good with game) will stop texting her after the date is set up. This is not a good move because, like everything else mentioned before, the woman loses interest. Her chances of flaking out go up astronomically when you do something like this. A simple text of “How’s your day going, sweetheart? I hope it’s as much fun as mine is today :)” will suffice in that time between the day you set up the date and the day of it.
The first thing I will do if she does not answer my text, and I know (just by the way she was texting me before) that she definitely received it by now, is send a text like this, “K sounds good lol” or even one like this, “All right hun that sounds Awesome lol.” Both of these make light of the fact that she didn’t answer you. They are short and sweet. They do not blow it out of proportion that she ignored you. Instead, they make a joke out of it, and they usually work to get a response.