Hello nico - I personally agree with your approach and am doing the same. I'm doing this for 4 weeks. Then I might decide to do another 4 weeks. And I'm carrying on like that.
If that makes it easier to 'mentally' manage, then why not? you're not obliged to sign up for a minimum period, no-one's telling you what to do, and the only person you have any obligation or commitment to, is you.
'Months and months' can sound like a very long time when you're struggling, so giving it a finite time limit is (as far as I'm concerned) is perfectly justifiable. Works for me, anyway. As does the 'one day at a time - anything is a plus' approach.
Whatever works for you.
And one last thing, I really struggled last week too and found it disheartening that all I read was 'how great it was' and 'finding it easy'. I felt like filth for assorted reasons, and thought it was just me. Well, remember it is an actually quite extreme thing to do, and not without side effects - esp in first week, and sometimes at random points thereafter. But rest assured, you are not alone.