TwinMOS gives data for USB pendrive here (min 10 000 r/w): http://www.twinmos.com/flash/flash_p_usb20_md_s1.htm
And for SD/USB combo here("min 100 000 erase cycles"): http://www.twinmos.com/flash/flash_p_usd2.htm

As many have experienced, flash drives may burn out fairly quickly if they are used as ordinary RAM, or a write-intensive harddisk.