How do you specify duration tasks, and what do you do to stop yourself from gaming the task (I.e., waiting it out with low effort) if you really don’t feel like doing it today?

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.