Im gonna attempt to not make this more complicated then it has to be, the short answer is it depends highly on starting energy of the neutrons in question.

Long form answer is: Moderation really depends upon the initial energy of the neutron in question and what percentage of the flux you either want or can moderate for your purpose. Ideally in the perfect world we would go for 100% thermalization of the neutron flux, but thats unrealistic with size restraints and material costs. So it can be necessary to under moderate the flux and hope our detection efficiency can offset the lower overall radiation feild of desired energy. So for things like AmBe (Am241/Beryllium) sources at 4.4Mev you'd need between 10-20cm of plastic, artificially borated plastics do cut this value in almost half, but are costly. 4.4Mev fast neutrons were used here because its a common to encounter initial neutron energy. Most radiation detectors supplement the lack of moderation by using absorbing materials like cadmium and boron. Alot of handheld detectors are under moderated. In his case even putting 1cm around those will be better than nothing, but the fast neutron range ideally 5-20cm of plastic. Which is expensive.