My first reaction was - just buy more disk space. It's cheap. But the context here does quite allow for that. It's pretty interesting to hear about development in a military environment.
Military multimedia development is contracted to 3 or four different companies who put development teams here on the base to research and develop the courseware using government provided computers, software and networks. Under their contracts everything becomes the property of the government once they receive tasking and begin developing a course (and I do literally mean 'everything'). It's pretty much set up so that each contractor has a 'folder' on the development network - they subdivide within that folder as necessary depending on how many courses they're working on, etc.
The govt prefers that while it is ok to work on your local machine, end of work day or finished items should be kept on the development network. At the moment we use VSS to manage assets - it allows check in and check out and tracks file use. We are just about to move to Adobe's Version Cue.
Additionally, once a course is completed and delivered it's not removed from the network storage - the govt takes over the maintenance and any updating of the course - and on top of that we're required to maintain backward versions of the course. So three months from now if a course is revised in some way we're required to keep the old version as well as the new one. so even our backup storage becomes quite complicated! (requirements state we're to keep 3 back)
Unfortunately for us, the military isn't able to simply add X TB to its storage design. They are required to predict growth, show usage and justify that addition.
So, I have to supply them with storage figures and growth predictions.
I've used various approaches and have based things off of prior work. But in most cases, it is definitely pure guesswork.
I should also add that this is mostly a situation applicable to the development side of the house. Once you have a finished product and it's been cleaned up and put in a 'run time' condition for the LMS, size (while still important) does not become that much of a storage issue.
X minutes of finished video * A MB +Any such formulas out there?
Y minutes of finished audio * B MB +
Z minutes total runtime * C MB