I usually give people time to respond in about a week or so - after so much time I mark them as unresponsive and file them accordingly in a folder. Sometimes, people are just too busy to respond within minutes or a couple of days (work, school, life, etc). And if I can't respond so many emails, I always try to make an update on facebook, to let everyone know that I'm still alive (it is where my portfolio is currently at and where I can post on the go for any news and updates).
Even with Etsy, I try to contact all my buyers to let them know what's going on and when to expect their shipment and/or try to make a shop update.
Some of the emails I get also end up in my spam folder, which I don't check very often, since it *is* a spam folder, but occasionally there is an email or two that isn't spam.