It used to be that you were supposed to keep your demo reel to no more than five minutes. Things have really changed in the past ten years or so. Now, it's really about them not being over two minutes. With agents and casting people getting huge amounts of demos sent to them, they simply don't have time to watch more than a couple of minutes.
And as someone who has talked to a lot of agents and casters, as well as made my own and other people's demos, I can tell you that those agents and casters feel they can get a good sense of your acting and your characters very quickly. It's true - if you've watched enough demos, you can tell within two scenes whether the actor is good or not.
With those things in mind, you will want to put your best stuff first. And I know that if you have a couple of great scenes that run about two minutes each, you may feel like you should put all of that in the demo. Don't! An agent doesn't need to see a two or three minute scene to "get" you.
Remember, a demo reel is just that - a demo. It's a business tool, like headshots. It's purpose is not to entertain agents and casters. It's so they can get a quick feel for your acting ability as well as the kind of characters you are best at portraying.
Remember what I said about keeping it short? Imagine this - you've got two long scenes that you're really proud of - now if the caster is only going to watch half of the first scene, then that second one never gets seen, right?
And because I've directed a bunch of stuff, I've received actor's demos with really long scenes. The worst are when the scene goes on and on and there's really no change in the character's emotion. An agent or caster doesn't need to see us be sad or angry or happy for two or three minutes. After 30 seconds, we get it - time to move on.
As an actor who has some good, long scenes on my computer, I know the feeling of having to edit them down to what seems like a too-short piece for my demo.
If you only have two long scenes, then make your demo one minute. Scenes should be between 15 and 30 seconds long. I know - it seems really short, but that's all the casters and agents need.
As for the rest: I open with a quick text card that says "Michael D. Nye" and under that "Demo Reel" Then at the end I put "Contact (my agent) and his phone number and email. And keep it short - if they need to write down the info they can pause the demo. Of course if you don't have an agent, just put your contact info.
If you have other questions about demo reels, drop me a note and I'll answer them.