It very much depends on the system, and also the story.
If I'm running a D&D-style game (I tend to run 3.5 or Pathfinder), our group tends to run those for a year or two, and usually I conclude the story around level 12-15. This is partially because the group likes to play a lot of different stories/systems, and also because of how D&D works at higher levels. Once you get into mid-high to high levels, the balance of the game gets more delicate. At very high levels, it's often a case of "he who acts first, wins." There's a decided number of save-or-die, or save-or-suck spells and abilities that come into play, and if you aren't very, very careful, you'll have multiple scenarios where someone has to sit out long parts of the encounter. If you're prepared and ready for that, more power to you, but unless in certain circumstances, I often find running those high-level games to be more work and less fun.
In games using the Cypher system, I've run right up to max tier (Tier 6) with no problems. That system allows for easier customization on the fly, and I have never felt the kind of pinch from max-tier powers as a GM that I have from high-level abilities as a DM.