It depends on flash memory technology, which depends on the manufacturer. Taken from the "SanDisk SD Card Product Manual":
SanDisk SD cards have an endurance
specification for each sector of
100,000 writes typical (reading a
logical sector is unlimited).
Therefore, extremely heavy use of the
card in cellular phones, personal
communicators, pagers and voice
recorders will use only a fraction of
the total endurance over the device’s
lifetime. For instance—it would take
over 10 years to wear out an area on
an SD Card based on a file of any size
(from 512 bytes to maximum capacity)
being rewritten 3 times per hour, 8
hours a day, 365 days per year.