[dfreidin]

There's no time limit, but a 60% vote can force him to stop.

http://en.wikipedia.org/wiki/Filibuster_in_the_United_States...

Edit: That's 60% of all Senators, not just those currently there.

[younata]

  Edit: That's 60% of all Senators, not just those currently there.

Also known as 60 Senators are required to vote to force him to stop.