How long does it take for an FAA licence to be delivered?
The FAA has, at least in the recent past, been backlogged with some forms and certificate issuance has been one of those items. My private pilot certificate took nearly 3 months to arrive (hence, my temporary certificate almost expired). I had to make several calls to my DPE (Designated Pilot Examiner), who then reached out to the FAA in Oklahoma City to see what was wrong. It turned out a computer glitch had erased my knowledge test report. After several times around, this was fixed and my certificate showed up within several weeks. In the USA they always send it via First Class Mail (uncertified, no tracking number). Last August (2016) my new certificate showing my instrument rating showed up just a month or so after the checkride, so much faster. If you have a question, don't hesitate to reach out to your DPE - they are very helpful and can get things moving again if they're stuck. Also, make sure you keep all test reports and other documents until you receive your certificate in case they somehow get lost in the system. And when you get your new certificate - don't forget to sign the back or it is not legally valid!

Regarding your interview - your temporary airman certificate is valid for all purposes until the expiration date. You can bring this to an interview as it is as valid as the plastic card. I don't know if a company is allowed to reject you because you only have a temporary certificate while your plastic one is being prepared - I'd look into this and maybe ask an attorney as that doesn't seem like it would be legal to me since the certificate is valid.

Cheers!