Depends on lots of stuff. Like how long is "a while" to text back? I really don't get nervous, I just assume they're busy.
Except with my girlfriend, she's always on her phone when we're driving or something, or watching a movie together, but she does take some time to text me back every now and again which makes me a little nervous, but I generally don't care. She's probably taking a nap, or with her dad somewhere.