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

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 (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

Martin Miles PDF presentation says at a given rate, which can be specified in using the rules engine.