Normally, cubes should be examined at 7 and 28 days. The exception may happen in special cases, like in the case of eliminating a concrete shutter securely before 7 days.
Nor 1 cube will be examined at 7 days and 2 cubes at 28 days. According to your needs, that may change. The design should be verified. The cubes are detached from the curing tank as well as dried and grit removed. The cubes are examined with a calibrated compression machine. It is accomplished inwardly by experienced personnel or by a certified test house. The cubes are examined on the face at a 90-degree angle to the casting face. The lowest compressive strength of concrete will be stipulated by the client/designer in a particular format.
Concrete Cube Weight
Unit weight (Density) of concrete=2400 Kg/cum
So, mass is required and the formula is Mass=Density X volume
Volume of cube=150mm X 150mm X 150mm
Mass=2400 X (0.15*0.15*0.15) = 8.1
Weight of cube in kg = 8.1
To learn more, watch this video tutorial