**When is the decay rate actually applied to the Contact Behaviour Profile? Daily, weekly, monthly?**

According to martin miles, and martin davies it is per interaction.

**Is that configurable somewhere?**

Martin Miles PDF presentation says `at a given rate`, which can be specified in using the rules engine. I'll update answer as to how this works, when I get a chance.

> A visitor’s behavior profile from their
previous visit can now influence
personalization rules in their current and
future visit.
Behavior Profiling are now stored both on
the Interaction and Contact.
Behavior Profile on the Contact can `decay`
at a given rate `per interaction`.

Source: [Martin Mile Presentation PDF][1] (page 17)

> Profiles items now have a `"Decay Rate"` field which specifies the percentage by which a contact's profile score should `decrease` at the start of `each interaction`. 

Source: [Martin Davies Blog Post][2]

  [1]: http://blog.martinmiles.net/file.axd?file=%2F2016%2F01%2FSitecore%208%201%20Technical%20Presentation.pdf "Martin Miles Presentation PDF"
  [2]: http://sitecoreskills.blogspot.co.uk/2015/10/behavioural-profiling-improvements-in.html