Changing habits is hard and therefore requires time. Not sure a 7 days trial is enough to see the results therefore the value of the application. I would prefer not to invest time using a new application if I do not have enough time to assess the benefits from the time I invested in.

Feels like 7 days is enough to get the idea of the application, its philosophy, add the first few things I want to accomplish. Not more.

Trial should end when one small goal has been accomplished so that the user could realise he/she has accomplished a goal she wouldn't have without the application, plus in less time/easier than expected.