Is there a property for a video’s length? I couldn’t find anything at: http://framerjs.com/docs/#videolayer.videolayer
Also, is there a good way to dig into things like this in the source?
The VideoLayer's .player property (used for things like .play or fastSeek (as shown in Framer Docs)), is an HTMLVideoElement. Therefore, you can use the HTML Audio/Video reference to view the properties you can use. In the case of duration, that would be layer.player.duration.
I started by checking Framer's source on GitHub (specifically VideoLayer: https://github.com/koenbok/Framer/blob/master/framer/VideoLayer.coffee). Once I found that the layer create's an HTML Video element, and adds it to @player, I hit the internet to look for options.