I used to think of it in terms of "number of lines" but then I found a task that worked quite well in a longer bash script (creating a .deb file). In fact, it worked better than if it were a python script, because all the commands were right there.
Now my metric is, "if it needs a function, or even a complex while loop，switch to perl/ruby"
I think that what I would tell new people would be "more than ten lines, OR using more than two variables, OR any flow control constructs". Bash can make even simple conditional statements difficult, and while my rule is absurdly restrictive, Bash can still be amazingly useful within those bounds. Probably mine is the lower bound for "anything less complex than this is fine to write in Bash" and yours is "anything more complex than this should definitely not be written in Bash", with the middle ground being "have a good reason why Bash is the best tool for the job".
Now my metric is, "if it needs a function, or even a complex while loop，switch to perl/ruby"