I agree completely. Any well designed embedded controller includes a watchdog circuit of some kind. You cannot depend upon perfect code or perfect hardware. We don't live in a perfect world.

In general the answer to the poster's question is that you can have a functional embedded system powered on for years (I know of many of my own design, deployed in systems that remain powered on indefinitely). Embedded systems are not necessarily like the upiquitous PC that must be rebooted periodically to clear out creeping memory allocations resulting from poor application design. With that said, however, embedded systems are not immune to bad design or poorly crafted memory allocation schemes. Utlimately the long term reliability of a system is mostly due to the quality of the code and the quality of the hardware design. Individual chips like the CPU can be powered on relatively indefinitely.

For reference, here's a link to a good treatise on watchdog design.
http://www.ganssle.com/watchdogs.htm
Regards,
Steve C.