I'll be completely honest and say that I don't think this one can be nailed down by anyone but the person with whom you are texting. There is no rule that can be set except by yours and his standards IMO. Therefore the person you should be asking is the person you are texting to. Some may not mind, like me, texts at any time because frankly, I keep my phone on silent 99% of the time and it doesn't really interrupt my day. Plus I text back when I feel like it and although most times that's as soon as I see it, and other times when I have a moment to pause, think and reply. I only use myself as one example, that it probably wouldn't matter with me, and to make a point that there are any number of texting habits out there, and no one here knows his.

Ask him.