Hard drives have moving parts so sooner than later they will wear out. If you have a 120GB SLC SSD and you write 100GB a day, it will last for 120 000 days, which is equal to 329 years. Write 1TB a day and it will still last for 33 years. However, NANDs lose their charge after about 10 years so that will happen before you wear the NANDs out by writing.
All else being equal, an SSD will _vastly_ outlive a mechanical drive, outside of corner cases involving massive amounts of data being constantly written.
That's for SLC, drsmithy originally referred to the much more common MLC, which will only last 5,000 writes. Don't forget also, that you're assuming the drive will only write once to each block for every 120gb written. In the real world, that's far from the case. If you have a 100gb drive that's 70% full and you write 10gb per day, each block will be written to once every 3 days.
It's not as simple as it first seems though.
If the ATA controller writes just 1kb to a SSD, a
whole cell (usually 128kb) has to be erased and rewritten. This means that for a 20gb drive, you could make as few as (20,000,000kb/128kb) 156,250 1kb writes to the drive to use one erase/write cycle on every cell. That's just 156 megabytes to use up one write/erase cycle for every block on the drive. You might think that no-one writes 1kb, skips 127kb, then writes 1kb again all over the drive. That's very true, but the wear levelling algorithm writes randomly when small pieces of data (less than 128kb) are sent to it, using any empty cell it feels like. In this extreme case, if you wrote 2gb per day in small 10kb chunks you'd need to erase 200,000 separate cells (2000000kb/10kb). This would use the same number of cycles as writing 25gb of data to the drive in large 128kb chunks. The drive has to erase and re-write all 128kb per cell each time 1kb in that cell is changed. In practise, the drive has a hefty RAM cache to mitigate this problem, but it does still happen to some extent.
The wear levelling algorithm isn't as perfect as some like to believe, my OCZ Vertex SSD is less than a year old, but it's already written to some cells over 1,500 times. I've got virtual memory turned off, and I rarely write to the drive. The drive reports that it's got about 70% lifetime left after less than a year. That gives it a total lifetime of around 3 years. If it was a cache drive, it would be overwritten almost continually with little bits of data, so its lifetime would be much less. If a SSD is often used for lots of (even small) writes, whether theoretical calculations disprove it or not, it will last much less time than a HDD.
If you're doing HD video editing, or even playing a few different games, the cache drive may well be overwritten several times a day. Portal 2 is about 6gb, WoW is 20gb+. A 20gb MLC drive wouldn't last long at all if used with the Z68's caching mode. If overwritten fully twice a day with 128kb chunks, that gives it 7 years life in theory. If overwritten randomly, the amount of data sent by the ATA controller may be smaller, causing the problem detailed above.
