It should last for several years if never used... The main point is that there's a finite number of times that you can write to flash memory before it gives out. Currently that figure is around the 10,000 mark IIRC. So, odds are, you'll have replaced the thing long before reaching that figure. It shouldn't affect reading, so you could just copy everything off if you start getting write errors.
The long and short of it is, you're far more likely to have replaced the memory stick/card for one with more capacity long before you reach a point where your data is in danger on the old one.
What is the life expectancy of data on a memory stick?