I can't give you a specific example but it is a "generally...

I can't give you a specific example but it is a "generally accepted" practice. Effectively its a reflection of payback. It's rare to do any major project with a payback of more than 5 years. Except government infrastructure projects.
Think about it, if you are buying something, you want a return on investment . Apart from having a positive ROI, the payback period is important. If you can't pay back the investment within 5 years using an appropriate discount rate, then on a pure numbers basis, you wouldn't embark on the project.
We may get lucky with a suitor willing to pay overs but they all know how desperate Vocus is so that will have an impact.