Period Costs on long-standing (evergreen) programs

Hello community!

My company is beginning the process of using period costs in our programs - and while the logistics makes sense for batch & blast and date specific events (like webinars)  - I'm a bit at a loss on how to implement it for long-standing programs.  Not for the cost but for the month that I'm to assign it to - Do I need to add in a PC every single month?

• For instance, our website brochure programs - those download/programs are always on the site with no date attached. How should I define the period costs? 
• For on-demand programs that are evergreen - again, how should I establish that period costs?
• What about for nurture programs?

I've read through all of the Marketo documentation, which is a great "how to" but doesn't cover these specific scenarios.  

What are you guys doing out there for such items?  

Thank you, 

A