Interesting question, I'm pretty sure they'll last longer than the useful life of the product itself, and longer than a SSD in a normal computer as the iPads memory is likely to see fewer Read/Write cycles than a normal computer.
Generally speaking, isn't there two types of flash memory, and only the high-grade is used within computing devices for storage? And the low-grade stuff ends-up in things like USB sticks? This is one reason why tablets are so expensive these storage chips cost much more than the equivalent USB sticks, albeit they seem the same.
So while USB memory sticks and memory cards will fail within a few years (or even less if used daily), this probably won't happen with storage used as primary storage within computing devices.
This PDF seems to explain:
I'd honestly worry about every other component before I'd worry about memory failure. Flash reliability is variable but at the minimum should be good for 100,000 program/erase cycles up to 1,000,000 program/erase cycles. Keep in mind this is on a per block basis so just because you've reached the limit in one block doesn't mean the entire memory is shot.
I've wondered about this as well. Seems that the software on the iPad will be outdated long before the hardware becomes unusable. Would not updating the software (ie no new apps, not updating current apps, not updating iOS) alleviate this and make the hardware usable for a longer time period?