It really depends on the show. If a show is arc-heavy like nuBSG it's very hard for a show to sustain momentum, especially of the writers don't have a long-term strategy, which of course is hard for writers to do when we live in a world where most shows are voted off the island after 13 episodes (run the stats and you'll find the number of shows that actually survive to a second season is actually in the minority; drop the percentage even further if it's a genre show on mainstream network TV - Fringe and Chuck have both defied the odds and X-Files' 9-year and Smallville's 10-year runs were remarkable, though you'll find many saying those shows should have ended after 4-5 years). Shows that do tend to last are those that don't have ongoing arcs and rely on standalones for the most part - the Star Treks qualify for this, and of course Law & Order, the CSIs, and of course Doctor Who. That doesn't mean there can't be "internal arcs" lasting a few stories or even a whole season, but compare to nuBSG or Lost which attempted to tell a single story have multiple years.
On the other hand, soap operas run with their arcs for years, sometimes decades. So I think it really depends on the audience, and how the show is handled, and if they have a game plan. Babylon 5 had a game plan, but even it ran into trouble because it lost its lead actor after the first year, and got cancelled with one year to go, so they had to change the plan. Farscape got screwed royally.
The thing that annoys me is when a show is compared with another in a franchise and declared a failure because it didn't last as long. Forget any subjective quality issues, Enterprise was in no way a failure - it lasted 4 full seasons. Those who call it a failure based on length criteria must therefore declare Farscape, Firefly, etc failures as well. Most SF producers would kill to get 4 seasons.