Swinging this back on-topic, I think that a minimum of three years can be considered a good run...especially by today's television standards. I am speaking on a totally objective level when I say this. There must be an audience out there who enjoys Supermarket Sweep and Shop 'Til You Drop, so that is all I can say about that.
I feel that if a show ran for two years, it was watchable but did not have enough appeal to go for thirds. Shows like Blockbusters (Cullen), The All-New Let's Make a Deal, and Bullseye fall under that category.
Many runs also depends on the taste of the audience to whom it is presented. For example, the Australian version of $ale of the Century went more than a decade with the Shopping Round, while we only had it for two years (One year on NBC, and one year on syndication...maybe because NBC needed a budget slash, but that is for another post). At the same time, their version of The Price is Right only had two three-year runs, with another run currently starting.
In summary, 2 years is notable, but three years is nice.
The Inquisitive One