What would be an acceptable limit ? 10 years ? in that span you're free to plan long project, even raise children in the same place long enough they can develop habits, friends, without too much stress.
There is no acceptable limit. It should be for life, or longer.
Remember that no rule nor policy is for ever; you always have politics. The new party in power might change everything, always.
A test is a test is a test.
And, in my opinion, the test would be useless if it had a time limit attached to it.
I think we could get some insights even with a broken test. But it has to be made by open minded scientists other wise people will interpret it as final truth.
Problem is that a "good economy" is a subjective idea. It won't matter if open-minded scientists review it 'cause people are going to interpret the results how ever they want anyways.
Edit: I'm not really aiming to contradict you. I would like to see it analyzed in an unbiased manner.
I think it's completely useless. The only results I care about are in the long term (30+ years). This is essentially a 2 year salary giveaway. Not even close to UBI.
However this test is not testing the actual idea of UBI.