How much time would pass before overwriting data on iPod/iPhone flash memory results in failure?
Flash controllers generally degrade gracefully, so once a block has failed, it no longer is usable, so you would expect a slow, gradual shrinking of the usable space over time. Once the majority of the blocks start reaching the limit on write cycles (you need to store different data - not just the same bits to make real change on most flash controllers) then this reduction of space increases more linearly with time / write cycles.

So - it will be quick to not store the entire file, but a much longer time until you can't write to 50% of the drive.