Watch the Concrete's Appearance
At least one contractor recommends stamping when the concrete's surface loses its sheen. Then, according to the Stamped Concrete website, you'll stamp with light pressure until the concrete gets harder. On a larger project, your first few stamps will need to be tamped by hand, or your lightest crew member can stand on them. As the concrete dries, use harder pressure or heavier people.
Test the Concrete's Firmness
One manufacturer recommends a different approach. An Advanced Surfaces instructional video says to wait until you can stand directly on the stamp without sinking in or making a deep depression. This approach leaves less workable time, so the manufacturer says less experienced installers should do small projects of 400 square feet or less.
Regardless of your method, timing is crucial. The concrete must be firm enough to withstand stamping, dry enough to not stick to the stamp and workable enough to take a good stamp. For these reasons, at least one concrete expert says concrete stamping is not a do-it-yourself job. Chris Sullivan of Concrete Network says concrete mistakes and repairs are too complicated and expensive to risk.