How much time we have to wait mostly depends on the type of cement used (high early strength or just regular cement), temperature, and humidity. A high early strength cement mix that is curing in a warm shop with a high relative humidity can often be polished as soon as the next day. Regular cement mix in a cold shop with low humidity can take up to a week to harden sufficiently.
If you try to polish green (soft) concrete, it only creates problems. Polishing actually removes material from the surface of the countertop, which is made up of stone, sand, and cement. Sand and stone are naturally hard materials but if the cement component of that mix is still soft it will be worn away much faster than the sand and stone creating an unlevel surface and actually ripping some of the stone and sand right off leaving pockets behind.
Not a pretty finish and since this project also included glass it was important to let the cement part of the concrete mix harden sufficiently before beginning so we could end up with a smooth surface.