Assume message is non-urgent, ie not 'where are you, I'm at the (place),' and not work-based either. So just casual chat.

Do you stress if they take too long? How long is too long?

Do you take ages to respond but expect fast responses from others? Or reply quickly to people who take ages?
I think a few days is a good timescale, but it varies depending on interest in given topic.

WBU?