I usually send a second email after two days, just in case it went to spam, got lost in cyberspace, or it was read and "meant to get back to then stuff happened". If I haven't heard back by the following week, I either call (if I REAAALLYYY want that specific snake), or write it off as business they don't want. Now, if it was just an email asking for information (not previous or future purchase-related), I'm happy to get a response in a week or two.