2-3 weeks are generally the more common timeframes I see people using, but a lot of that depends on the length of your sales cycle as well. If you're B2C, could be shorter. Long B2B could be longer.
This is one of those classic things you probably want to test. I know many people wait up-to-three weeks, but I am old school guy (I grew up when follow up campaigns were handled vis direct snail mail) and I usally do a follow up email a week later since most resposnes from your intiial email will come in the first 48 hours or so. Can't overstate the importance of testing this out and as your user base diversifies you might see different responses based on segment.