From the atmega328 datasheet:
- Data Retention
Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over 20 years at 85°C or 100 years at 25°C.
Also: http://support.atmel.com/bin/customer.exe?=&action=viewKbEntry&id=266
http://www.atmel.com/about/quality/reliability.aspx