A big issue with any analysis of happiness is that it's rated on a "capped" scale. For example they ask you "how happy are you 1-10". However income is "uncapped". That means there will always inherently be a sublinear relationship between the two in any analysis, because you are compressing the top end of happiness.
I think for most people amounts up to something like 200-500k/yr (depending on COL) would provide increased happiness. Basically, if you ever have to worry about not having enough money, you could stand to make more.
Of course that doesn't mean it's necessarily worthwhile to work more to achieve that, that would be a personal decision you have to make yourself.