I have one clarification -
First the definitions-
User-based: Recommend items by finding similar users. This is often harder to scale because of the dynamic nature of users.
Item-based: Calculate similarity between items and make recommendations. Items usually don't change much, so this often can be computed offline.
Now the question - in item based CF the similarity between items are tracked via user behavior - since user behavior is changing will it not impact the similarity between items?