When you start a campaign, how closely do you plan for when it will end?
It depends. If I am running a dungeon crawl -- such as my current campaign is -- I'll have a general idea of what the end game will be as well as an idea of what encounters the party will face along the way.
However, if I am running a wilderness exploration-based campaign then I don't have any idea of how the campaign end. It will run until the PCs die, retire, or if the players are ready to move on to a new campaign.
Do you expect the PC's will get to a certain level? Do you run for a set number of months?
Again, it depends. My dungeon crawl, for example, I am expecting that the party will be in the level 7-8 range by the time they reach the nadir. They average about four to five encounters a session and we play monthly so I am looking at a two year campaign. I didn't intend for it to happen this way, though.
Generally, I play under the expectation that the campaign will end when it ends (i.e. the PCs die, retire, or the players are ready to move on to a new campaign).
Do you create at least one Uber-Plot and when that is complete the campaign is over?
Not plot as in the LotR's journey to Mount Doom and toss in the One Ring sense. I build games with NPCs who have agendas with which the players may (or may not) find themselves entwined.
My dungeon crawl, for example, has an over-arching goal of explore the dungeon. Inside the dungeon there are numerous factions that the PCs can interact with. Depending on the decisions the PCs make they may find themselves with enemies whose machinations they are trying to thwart; allies who have goals that they would like to see accomplished; or neutral entities whose agendas they are indifferent to.
Understanding the desires and tendencies of my players does allow me some ability to predict the road my players will take; though they do tend to surprise me from time-to-time.