How many days after the job interview should I wait to send an email if I dont hear back from them?

Depends on the job. If it's entry level like fast food or retail, you should hear back with in a week or two. 

At least a day after an interview, you should send a thank you note. Call them if you can, don't email. Ask if they're still interested in moving forward with you for the position. Also ask any questions you may have about the job you applied for. 

One to 2 weeks. 

At the interview they would normally tell you how long before they will contact you and if they don't you ask... if they/you failed to do that then 'normally' a decision is made within 24 hours so give it 2-3 days

You shouldn't wait at all.  Send your email soon after the interview, on the SAME day.

A follow-up thank you email after a day or two is good along with a note showing them your interest in the position. Many candidates use a spray and pray approach so it's always good to show the employer that you're serious about 'their' job.