Hey Sails,
For things like this I usually try a simpler goal first, manually entering a +1 on days that I actually do the thing. The minutes often take care of themselves.
How are you taking the video? That might give folks more ideas about how to automate it.
If it ends up on your computer through the magic of Dropbox or iCloud, you might be able to use something like Hazel to automate a script that extracts the length and adds a datapoint to Beeminder.