Are long free trials actually killing user activation?

We thought giving users a 30-day free trial would give them plenty of time to explore everything and fall in love with the app. Instead, most signed up, poked around on day one, forgot we existed, and then churned when the automated reminder email went out. When we shortened the trail to 7 days activation actually shot up because it created a sense of focus.

What length of free trial or sandbox experience has worked best for getting users to their "aha" moment?