Both work. They are just answers to different questions, and picking the wrong one is the most common reason a challenge dies in week two.
The short answer
Pick 7 days if you are testing whether you can do the thing at all. Pick 30 if you already know you can and you want it to stick.
A 7 day challenge proves you can start. A 30 day challenge is long enough that life gets in the way at least once, which is the part that actually teaches you something.
What 7 days is good at
- Finding out if you hate it. Seven days is cheap. If you loathe cold showers, you will know by Wednesday and you have lost almost nothing.
- Getting a win when you need one. If the last three things you started fell over, finishing something small matters more than finishing something impressive.
- Testing the logistics. Does the time of day work? Do you have the kit? A week answers that without committing a month to it.
What 30 days is good at
- Surviving a bad week. In seven days you can white-knuckle through. In thirty you cannot, so you have to build something that fits your real life.
- Making the thing normal. The point is not the thirtieth day. It is that somewhere around day eighteen you stop negotiating with yourself about it.
- Showing you the shape of your misses. One skipped day tells you nothing. Three skipped Thursdays tells you your Thursdays are the problem.
The 30 day myth worth ignoring
You will read that it takes 21 or 30 days to form a habit. It does not, reliably. The research people quote for this varies enormously by person and by habit, and picking thirty is closer to a convention than a finding.
Thirty days is a useful length because it is long enough to be interrupted and short enough to see the end from the start. That is a good reason. "Your brain rewires on day thirty" is not.
How to choose, in one question
Ask what would count as a success.
If the honest answer is "proving to myself I can do this at all", take 7 days. If it is "having this still be true next month", take 30. Do not take 30 to impress yourself and then quit on day nine, because a failed 30 is worse than a finished 7. It teaches you that you do not finish things, which is exactly the belief you were trying to change.
Do the 7 first if you are unsure
Nothing stops you running a 7 day challenge and then starting a 30 with what you learned. That is usually the better order, and almost nobody does it because a week feels unambitious. A week that you finish beats a month that you abandon.
The bit that decides it either way
Neither length survives a tracker that punishes you. If missing day 12 wipes your progress back to zero, the 30 day version becomes a 12 day version with a bad ending, and you will not start a third one.
Whatever you use, make sure a missed day costs you that day and nothing else.
How the app does it
Reveal the Dream gives you 7 and 30 day journeys free, so you can run the short one first and the long one after without paying to find out which suits you. Custom lengths are a Premium feature, for when you know a goal needs 45 days or 90.
Miss a day and nothing resets. That day's piece of your photo stays covered and the journey carries on, which means a 30 day challenge is still a 30 day challenge on day 31 even if you lost a Thursday in the middle.