A lot of units use 1/2 inch or in many cases less. Upgrading to 5/8" makes a big difference, and installing another 1/2" on top of that gives impressive noise dampening via additional mass. As does fiberglass insulation, and filling gaps around the edges. If you don't have to meet firewall code for certain walls, you can go a lot thinner and noise reduction goes down dramatically. We had an apartment that was converted from condos and noise dampening was significant. Noise was a major consideration (closely after neighborhood) when doing selection.