In answer to the above question (in my case), around 16 months. I'd been running a collection of internal services (BIND, Bitlbee, Openfire, rsync, Hamachi, Apache, etc.) on a single Raspberry Pi.
The SD card held up for roughly 16 months before the system started acting wonky. Wonky, as in some services would start, others wouldn't. There was no rhyme or reason while troubleshooting. Finally I gave up and tried an install, using a new SD card. This appears to have fixed it as the system is operating normally again.
The pain caused by the above was minimal, but only because I had backups. Recovery took only a couple evenings work. I shudder to think about what I'd still be doing without those backups.