Every flooring (VCT, epoxy, carpet, ceramic, etc.) manufacturer is going to say to wait at least 28 days after concrete is placed or when the floor passes a moisture test - whichever comes later. They all say to wait as long as possible and prefer around 60 days but 28 will usually be allowed.
There are a few ways to test for moisture. First (and most expensive option) is to buy an electronic moisture meter. This is overkill and unneccessary for the average DIY unless you have loads of extra money and want to buy a cool little tool.
Second option is to take plastic sheeting (a cut up garbage bag works too) and tape 2' x 2' squares to the slab with duct tape for at least 24 hours. At the end of 24 hours, remove the tape and plastic. If the square that was covered is damp or darker in color, you have moisture being transmitted through the slab. I would do this in at least two or three places throughout the slab to make sure it is curing evenly.
There are several things that can be done to help the concrete cure sooner/better without damaging the finished product.
1. If the building isn't up yet, tarp the entire site prior to the pour if rain or snow is in the forecast. Any water that gets into the subgrade has to get out somehow (most likely through your slab).
2. Install a vapor barrier. It keeps ground moisture at bay for the most part assuming its installed properly and not torn during the pour.
3. Cover the slab with burlap and keep it damp during the first few days of the cure. It seems counter productive but concrete cures by hydration, a chemical process that needs water to completely hydrate the lime in the cement. It also keeps the surface from curing a lot faster than the lower portions of the slab. Curing too quickly and unevenly leads to cracking.
4. If you have the building up, run an HVAC system.
