First, a little background:
Flash drives (those little memory devices that you connect in to a USB port and you can copy files like it was a disk - but they have no moving parts) store data by shoving electrons into a storage 'cell', then keeping them there (or draining the electrons out of a storage cell, and keeping stray electrons out). So the "data retention" is controlled by how long the electrons stay put; they "leak out"(or "leak in") over time. How long that takes is determined almost exclusively by the quality of the manufacturing process and the temperature the flash drive is stored at. Also, there are two types of Flash devices: those that store one value per cell ("a lot of electrons" or "very few electrons") called SLC (Single Level Cell) and multi-level devices, which store multiple values per cell ("a lot of electrons", "some electrons"," very few electrons") called MLC (Multi Level Cell). As you can guess, the MLC devices can lose the "correct value" for a cell easier than the SLC, since fewer electrons have to migrate to cause confusion.

How do we write to a memory cell? We shove electrons through an insulator, or pull them out! This causes the insulator to degrade, so all Flash devices have a maximum number of times you can write to a single memory address.

Now to the answer (if you are still awake!)
Good quality MLC devices will allow you to write to a given address about 10,000 times. So if you wrote the entire drive once a day, it would die after about 27 years. SLC can usually withstand 10 times the number of writes. Both types will generally have a data retention time of around 10 years if stored at room temperature.

The problem is that there has been a huge demand for these devices (since they are in everything from cell phones to cameras to iPods), so a lot of counterfeit devices are being seen. In other words, devices that do not meet the standards of the manufacturer are being stolen and then sold as "good" rather than being destroyed. In this case, the write cycles and data retention can be dramatically shorter - sometimes less than 10% of what they should be, withstanding as few as 500 write cycles and data retention times measured in months.

Since it is impossible to determine if you have a "high quality" or a "counterfeit" device, I suggest you do not use flash drives to archive important data, but only as a quick & easy way to move data from one system to another, or perhaps for short-term backups where you won't be too upset if you lose the data (since you have ready access to the same data someplace else).