MLC's have 10k+ Write Cycles
SLC's have 100,000k+ Write Cycles
A write cycle consists of...
512kb block of data on the drive. If you write 4k or 512kb that is a block (you can continue to write to the block til its filled up).
From there if you delete an item in said block, the block at an appropriate time will cache the whats in the block, delete the file and re-write the block to free up the space.
So as for how long the drive lasts? Depends on how full it is. How much things change on the drive, etc. The drive should write to every aviable block at least 1 time before it makes a second pass on the drive writing data. That continues on and on.
I did some calcs in another thread, about basically saying a 60Gig drive with 30Gig free space thats written and deleted constantly would take 8+ years if not mistaken to wear out that 30Gig cluster. Thats if you where writing 10Gigs to the drive per day. Realistically it would be more in the realm of 2.5 daily for normal use (surfing, playing games and little things), 5-10Gigs would be on days of installation of games (rarer but still out there).