Under 1 year, I have to think that something out of the ordinary happened and I would be inclined to ask about what happened. Did the company go under? Were there some crazy red flags that forced you to leave? And internally, I'd ask myself, is this person really picky? Do they have some red flag that keeps getting them let go? Etc.
I don't have an upper limit, but definitely a lower limit; 2 years. 1 year to get up to speed, 1 year to give the company a chance. I recognize that all companies have problems and I want to see progress being made on solving those problems. If, after 2 years, we're still talking about the same issues we faced when I started, then I'm going to start looking.
Curious why someone would place an upper limit on time served? If the work is good and the company is good, why leave?