As much as is neccesary to actually achieve the liberal utopia of equal opportunity, while adressing negative externalities as much as possible (yes I'm well aware that this will never happen, but shoot for the moon and all that). This will probably mean more taxes, but just as importantly is how we pay tax, and how and what we spend the money on. As an example, innheritance tax should in this case be higher, and so could a tax on greenhous gasses. What to spend it on is harder, but a simple way to spend would be an equal sized cash handout for every citzen (this could also potentially allow cuts to welfare programmes if the summs are large enough).
The reason I won't put an upper limit is simply because that limit will depend on which taxes we have and what it's spent on.
Checking out seems not to do much.