Something positive, for a change:

1. What are the best movies where the shortness of the time frame is essential to the film's concept? Immediately coming to mind are

Many Bond Films
The Bridge on the River Kwai
Taken
Attack the Block
The Warriors
Jaws
Close Encounters
ET
Minority Report
Nashville
Eyes Wide Shut
Resident Evil 1&2

2. The best movies where the lengthiness of the time frame is essential to the concept?

2001
A.I.
The Godfather
Munich
Resident Evil 3&4
Goodfellas
War Horse
Majesty and Casino Royale
Mission to Mars (a combination of both)

These are just the movies I happen to be thinking a lot about these days. I really want to know what others come up with.

MGames, Sounds interesting, but I need a better explaination of what you mean by length of timeframe. Like movies where a "fuse is lit" so-to-speak?

I don't understand.

1) High Noon, 3:10 to Yuma and DOA; The Andromeda Strain
2) All Quiet on the Western Front, Forrest Gump and Zelig; Driving Miss Daisy

There are a lot of films where the action takes place over a single night. American Graffiti and After Hours.
(1) 25th Hour, (2) The 25th Hour.

1) Zulu, Dirty Harry
2) Citizen Kane, It's a Wonderful Life, Scrooge

Yeah, fuse is lit is how it usually turns out, but stuff like After hours and American Graffiti is what I mean also. Though I guess American Graffiti is a fuse is lit movie too; 24 hours to make the most important decision of your life.

1) Die Hard, Inside Man, Collateral (all movies involving a heist or other crime where the movie picks up with the plan already being put into motion.)

12 Angry Men (similarly, it's a movie about a more extended amount of time, but set in a single evening. You learn all the backstory through discussion and monologue in a very short time-span, which is central to the concept, dealing with communication, perception, prejudice, etc.).

Run Lola Run. Interesting in that the time covered is shorter than the running time of the movie, so it has to be covered multiple times (in multiple universes). Really brilliant way to pore over and dissect a short amount of time in the characters' lives.

2) Tougher to nail down. Movies where the expansive time period covered is essential to the movie? My mind immediately goes to There Will Be Blood, but I find that the ending of the movie, when we really jump ahead in time and the movie swings for the fences, really doesn't work completely. We see the time we see in such detail that the bits we miss in flashing forward, where Plainview loses the rest of his humanity off-screen, make that development feel unearned a bit.

Shawshank Redemption: great movie that lets you really feel the development of two full decades. And not in any way a story that would work over a span of months--or even a few years.

Training Day
Nick of Time


and if they ever get around to doing the 24 movie, then we'll be able to add that too.

Those are all #1, an example of #2 I would say is The Tree of Life. And almost any biographical movie like J. Edgar, Ray, Walk the Line, etc.

Donnie Darko

1) The Hours This could probably go in both but I'm putting it here given the original inspiration Mrs. Dalloway was deliberately set over the course of one day.
Pan's Labarynth
25th Hour
3:10 to Yuma

2) Question: would pretty much any biopic be an example here? Given that those (usually) are set over several decades of a person's life.
Citizen Kane
Watchmen Another example that could go in both, as the present day sequence is "fuse is lit" while the back-story set over several decades is also important.

Not Lawrence of Arabia, which seems to only take place over a year or so. I think the perfect combination between the two categories is Groundhog Day.

So, I'd like to add:

Smokey and the Bandit -as Jerry Reed's song states, "We got a long way to go and a short time to get there".

Back to the Future Can there be any bigger pressure for time?! I've seen the Delorean vs lightning bolt finale dozens of times and I tense up every single time.

3'oclock High Tick tock until 3.

Planes, Trains and Automobiles Get home by Thanksgiving.

As for movies where there is no pressure of time but we are still witness to it's passage and effects:

Wyatt Earp
The Curious Case of Benjamin Button
and one of my favorite stories that was a passable movie, Tuck Everlasting

1) Speed and Memento

Chill Factor
Speed 2
30 Minutes Or Less

OK they're not the "best" films were time is essential, but I like them well enough.