Swordfighting in movies is entirely unrealistic (apart from some very, very few movies). The fights are choreographed to look good, not to be realistic - since realistic fights would most often be exactly one thing, as you mentioned: short. If you watch historical fencers (who practice techniques from old fencing manuals, most often the German books of the Liechtenauer school from the 14th to the 17th century), you will see that (much like modern fencing) actual engages are very short until a hit to the body is achieved, then the fighters separate and engage anew. If people were fighting without armor, as they often do in movies, this first hit would either most likely end the fight. But that's what armor was for: Fencing was not about parrying every blow, but keeping the enemy from reaching the unprotected areas of your armor. But that wouldn't look fancy in movies, so they don't bother.

Now, gladiator fights on the other hand could very well have looked like duels in movies (but not like in Gladiator!). Gladiatorial combat was not done with the aim to kill - it was a sport. It was about delivering a great show, about exciting the masses. So long exchange of blows with parrying and dodging would have been seen a lot.

On another note: Swords are not actually as heavy as you might think. A late medieval longsword could be wielded both one handed as well as two handed, was about 100 to 140 cm long (handle and blade) and would weigh in at only around one kilogram. That is really not a lot. So while one handed parrying of a leaping blow is rather unrealistic, it could maybe be done when grabbing the blade with the free hand and blocking the incoming blade with the part of the blade between the hands.

There are some very good responses here, but I would like to add, if you want to see what the fights should look like, look at scholagladiatoria on youtube.

The following links are videos of people who use actual techniques and tactics from historical manuals and descriptions.

This one shows sword vs. spear with different kinds of shields

Here is a video from a competition

This one uses weapons commonly depicted in film

Here is an amusing video which points out how bad movie swordfights often are

Scholagladiatoria and Lindybeige both include a lot of videos on techniques and criticisms of specific examples. Lindybeige also critiques a lot of cliches and common things depicted incorrectly in movies (the fact that a torch makes a room smell awful and ruins night vision, pointing out general historical inaccuracies and anachronistic things, that kind of stuff). Skallagrim also has a few videos talking about poor techniques in movies, and quite a few critiques of the weapons found in movies and video games.

A good movie to watch on this subject is "The Duelist." It's Ridlys Scott's first big movie, and has several types of Napoleonic era European sword fighting.

Generally, movie sword fights aren't very realistic. Most of them have the basics wrong. There is a lot of over swinging, where the attacker swings their sword and pushes too far into the attack, leaving themselves exposed. You're supposed to finish your strike in a guard so you can deal with the opponent's reaction. In the clip of Gladiator you have, their left hands are not where they should be. The left hand should be at you hip, behind your back, or clenched at your chest, just to have it out of the way. If your hand is forward, it is an easy target for your opponent, and you might accidentally cut your own arm/hand. As for how long fights were, it depended on the people fighting. Two skilled fighters could keep fighting for a little while, however, it's nothing like in movies. Your goal is to hit the opponent, not their sword. As for your first question, it could vary. The average European longsword weighs three to four pounds, which could be used effectively one handed if the user is moderately strong. If a strong blow came from above, it could possibly be blocked one handed, but only if the defending sword is angled towards the attacker so the attacking sword slides down and hits the quillons. A longsword held in one hand, angled perpendicularly to the offending sword, with the defending swordsman on the ground, wouldn't be able to defend against that kind of strike. But there are countless other factors that could change that scenario.

Edit: Spinning is also shown in movies a lot, and is a horrible maneuver in actual sword fighting. You're opening your whole back to your opponent, making it an easy target for them. Also, the direction you're striking from would be very obvious and the strike would be easy to parry. This leads me to another point I forgot to mention, telegraphing. You see a movie swordsman cock his arms back to prepare for a massive downward cleave, and you know he's going to strike from above. You can easily parry that strike.