How long can you read files from a flash drive if kept in a cool, dark room?

I think that with the current technologies the limiting factor will not be the amount of time the data will be able to be stored on the media without decaying, but the availability of readers. It is hard nowadays to read a 5 inch floppy disk, and almost impossible to read the stack of 50+ years old perfectly preserved punch cards left from my grandmothers education in the university.

So I think you should change the device with more modern every 10 years or so just to be sure.

Flash cells are... fairly stable. I'd say in the order of a decade or more. But if you want truly long-life media then you should look to OTP (one-time programmable) media such as PROM or EPROM. Unfortunately I'm not aware of any available in a USB format though, so you would probably have to drop down to using bare chips and a programmer.

And of course, don't forget the 3-copy rule.

All of the prior mentioned are good points, but if I were you, I would not trust only one copy of backup of this sensitive data especially on a flash device. I have had them fail in the past. Storing the data on a small external hard drive actually places the data on the magnetic platter, in this way EVEN if the hard drive fails, you can have experts repair or retrieve the data. Even in this case I would not leave it on the drive untouched for over 10 years, as hard drives are also susceptible to bit decay.

If you have the money your best options are to get a glass die made for imprinting either DVD's or CD's with the data you want, as this is an actual physical representation of your data. I also know that there is a company somewhere that will etch your binary data onto gold plates for the ultimate lifespan.

Tape backup is another time-tested solution except the readers and media can get expensive.

I personally use a device like this as because it stores the data with RAID 1, a mirror onto both hard-drives, so in case one fails, you can rebuild the data... and if both fail, the experts have twice the hard drives to retrieve data from.

Lastly, I would consider using a printed binary copy of your data. There is a “joke” (though functional) open-source project called PaperBack which includes such features as compression and redundancy that will allow you to print your data as 2D barcodes onto actual sheets of paper and store them in your safe.

The reason I mention all of these options is because lets say you store a flash drive in your safe, AND a burned cd AND a tape backup.. and your house burns down. The temperatures exposed to this safe will cause the flash drive, cd, tape, etc to melt and be useless (even with expert recovery people). Paper copies or etched glass will be more resilient to this type of damage.

Lastly I would put my data in the 'cloud', amazon offers data store with a free 5GB start I believe. Also a company called dropbox has a VERY convenient and cross platform syncing system with cloud storage starting free at 2GB