I am pretty old school. Texting for me is more about getting down to business. It is not I method I use to be social. It is about relaying the details of a subject I am already in discussion about. If I wanted to be social, that is what the phone is for.

I would never expect an immediate response. People might be driving. They might be at work. They might otherwise be engaged. They might be in conversation with other people. They might be at dinner (a whole other discussion here - the etiquette of when using your cell phone is acceptable). I would consider it rude to expect an immediate reply. However, I do expect promptness in that the individual ought to be responding back as soon as they are able. Not to respond at all shows poor etiquette.

Basically, I look at texting and messaging as a secondary means of communication. Life matters at hand by far take precedence than responding to a message. If it is important enough you feel an immediate reply is due then you should be phoning that person instead.