The wait time after the final interview should be no longer than 2 weeks. My opinion is that you should not try and inquire about the position before two weeks have passed since the last interview.
If they tell you that you will hear from them in 3 days then wait at least 5 days to make another inquiry...but only if you must. If they really want YOU then they will keep in touch and follow through.
You should send thank you notes to everyone who interviewed you within 24 hours of the interview. An email thank you will be more timely and I believe therefore more appropriate.
As everyone else on every job forum says, never stop looking until you get the offer. The offer can fall through for an infinite number of reasons.
Waiting is the absolute hardest thing to do after an interview, keep yourself busy and don't wait by the phone. You deserve to treat yourself better than this.
The employer has other things going on and may still be looking at other candidates. If you really felt like you nailed it then there is nothing more you can do.
There is no typical time it takes to hear back, if someone says there is, then they are not familiar with the process.