The late South Carolina Senator Strom Thurmond (S.C.) was so opposed to the Civil Rights Act of 1957 that before it finally passed, he filibustered it. But this was no ordinary rant. Thurmond held the floor from 8:54 pm on August 28 until 9:12 pm the next day. During the filibuster, he read aloud the Declaration of Independence and the Bill of Rights, among other historical documents.
Of course, Senate rules allow members to hold the floor as long as they can stand and remain talking. Floor speeches don't even have to be relevant to the matter at hand, as evidenced by Ted Cruz's reading of Dr. Seuss' Green Eggs and Ham in its entirety on the Senate floor Tuesday night.
However, Cruz is not really filibustering. The Senate is currently considering a procedural resolution on whether to debate Continuing Resolution 59, which was passed by the House. Each side of the aisle has been allotted 15 hours of speaking time and no more. So if Cruz hits the 15 hour mark for the GOP side, he must stop.
CR 59 is a Republican-passed resolution that would fund the federal government until December 15, but would defund Obamacare. That provision will be dead on arrival in the Democratic-controlled Senate.