When you start a campaign, how closely do you plan for when it will end?
Depends. Some I set to run for a fixed number of sessions, or a fixed length of time.
Most often, I am a tad more vague, and aim for short, medium, or long. I don't aim for a certain level, if only because that cutoff only makes sense for D&D, not RPGs in general.
Do you create at least one Uber-Plot and when that is complete the campaign is over?
I don't usually use single uber-plots for long-running games. No plan survives contact with the real world, and the longer the plan, the less likely it is to survive. I tend to do it in chunks, at the end of any one chunk, the game could end, but they chunks feed into each other neatly. From outside it (hopefully) looks like an uber-plot, but it isn't designed that way.
For D&D, for example, I might set up a major plot for the heroic tier, and have some ideas for continuation into later tiers. When they're getting late in the heroic tier, I'd probably start laying groundwork for the next tier.
For my current Deadlands game, I am more thinking in terms of the hooks my players gave me in their backgrounds. I'm running a number of short and medium length plots. At any given time, I have woven at least one (and usually more) individual player hooks into what they're doing.
Eventually, all the hooks they gave me will have run their courses. At that point, the characters have played through the personal stories the players started with, and at that point it'll probably time to move things on to some interesting/climactic end.