Three to four is my limit. That's based on my personal desire to balance writing time against personal time - yes, I count them separately. I can take from a day or two to a week or so to respond, because my work hours, while more-or-less fixed, are always variable - the longer I spend at work (something that happens all too often) means I have less time at home to do "me" things.
Any more and I lose track of what I'm doing.