It doesn't, not really, but there will come a time when it will simply not be worth keeping it up. Eventually the solar panels will not produce enough power, some airtight joint will break, or something similar. Right now the station is planned to be used until 2024. There is a study going on to see if it can last until 2028. The things that they are the most worried about are:

• Thermal cycling abuse- It seems likely this won't be an issue from studies so far.
• Solar panels- By the year 2028, the solar panels will have degraded about 30%. This might be an issue, and is the most likely item to cause problems.
• Electronics are out of date- Might need some fairly major updates. But this could be avoided if required.
• Cost- The ISS costs NASA bout $3.5 billion to maintain per year. That money could go elsewhere that might be more productive.

The bottom line is, I expect that the ISS will continue to be in use for as long as it makes sense to keep it operating. It cost a large amount of money to get it up there, and I expect until the station could be replaced, it will remain up there.

Of course, this could all be mute. If SpaceX managed to get the BFR working, it will essentially be the size of the space station. That would reduce the operating cost of the ISS somewhat, but it would also make it redundant. There are other companies talking about building their own space stations as well. Mars will likely be a major target then.

It doesn't, not really, but there will come a time when it will simply not be worth keeping it up. Eventually the solar panels will not produce enough power, some airtight joint will break, or something similar. Right now the station is planned to be used until 2024. There is a study going on to see if it can last until 2028. The things that they are the most worried about are:

• Thermal cycling abuse- It seems likely this won't be an issue from studies so far.
• Solar panels- By the year 2028, the solar panels will have degraded about 30%. This might be an issue, and is the most likely item to cause problems.
• Electronics are out of date- Might need some fairly major updates. But this could be avoided if required.
• Cost- The ISS costs NASA bout $3.5 billion to maintain per year. That money could go elsewhere that might be more productive.

The bottom line is, I expect that the ISS will continue to be in use for as long as it makes sense to keep it operating. It cost a large amount of money to get it up there, and I expect until the station could be replaced, it will remain up there.

Of course, this could all be moot. If SpaceX managed to get the BFR working, it will essentially be the size of the space station. That would reduce the operating cost of the ISS somewhat, but it would also make it redundant. There are other companies talking about building their own space stations as well. Mars will likely be a major target then.