How low do taxes need to be before trickle down begins to work?

As you may have heard, the super-rich have squirreled away $21 trillion – described as more than the economy of the USA and Japan combined – in tax havens. Had taxes been higher, some of that might have gone to needed spending or at least paying down the deficit. As it stands now, it’s just sitting in accounts accumulating.

Under the trickle down “theory,” how low will taxes need to be before it starts being spent?