Type 'Arrhenius' in search machine and you will find a good link to failure rate calculation theory (Arrhenius equation, chi square distribution, etc.)

It's important for you to use actual reliability data of your chip. Then you can estimate failure rate for each certain ambient temperature.

By the way: ATMEL chips are state-of-the-art in respect to 'data retention'!!

Kai

PS. Didn't want you to feel stupid, but lazy {grin}