Burned DVDs last between 5-10 years, depending on the quality of the media, as well as storage temperature, humidity, handling, etc. After that, data begins to "evaporate".

Flash memory depends more on write/erase cycles. Infrequently used media can theoretically last for decades, and is much more resilient with respect to storage and handling. Then again, in 25 years, how easy will it be to find a USB reader?