As a general rule that is courteous and fair to both parties...allow 5-7 business days to pass and then conduct a follow up.
If you have an email address, send a very BRIEF yet upbeat email thanking them for the interview, excited to join the team and looking forward to hear from you soon email within 24 hours of the interview.
Read it OUT LOUD several times before hitting send and I would even suggest having a friend not only read it but read it out loud to you.
The above is great advice...one day I shall publish all the emails, letters and replies I have collected over the years. I am often embarrassed for our profession and humanity in general!