I’d say for twice the expected usage time of the original packaging.

Say on average someone goes through the original bottle once a month (yes that is way too much usage, just as a hypothetical), they should be required to have it prominently displayed on the new label that the new size is estimated to last x% less time(or more time) than the previous packaging, for 2 months minimum.

I’m cases of recipe changes I think it should be required to have that on the packaging as well. I could see a soap mfg just diluting the soap to meet the volume, and bypass the requirement.