Is International Driver Permit only valid for one year?

Yes, correct, an International Driver Permit is only valid one year.
The permit you got in 2014 should have an expiry date on it, one year from when you got it.

The IDP is a certified translation of your licence. For the benefit of any police officer who needs to know, it certifies that your licence is a valid licence and for what classes of vehicles it is a valid. You need to present both your licence and the IDP to the police if required.
It is only valid for a year, because your licence could have been withdrawn in the meantime.

Thank you Chris for a detailed answer!

Actually it can be less than one year depending on the expiration date of you US drivers license. If you US license expires in less than one year, the IDP will have the same expiration date as your US license.

It's actually 1 year from the date of validity, rather than the date you got it. Last year I got an IDP in March, which was made valid from April 22, the date I was leaving. So now I can use it again this year, as I'll be returning just before April 22.

Thanks Irv and Robert, both your experiences are worth noted.