I find that if I continuously wait it out with low effort that it is too long a duration. I will pay attention to the time left and at what time I get bored. Then adjust the time accordingly. So for meditation I originally had 20 minutes. I found myself getting bored and falling out of it around 12-15 minute mark. So I shortened the duration of the habit to match. In reverse when I want to do something longer I add 5 minutes to it and see how that feels. It's whatever works to have you want to succeed in that habit and get you into it on a daily basis. You can always add and remove time. There is no right or wrong here. Just what you need to succeed.