Many factors determine the true shielding effectiveness of a shielding tape, including type and thickness of foil, adhesive type, the intimacy of contact, smoothness of application surface, strength and frequency of the EMI signal, etc.
However, using standard tests and fixtures, it is possible to determine a value for the attenuation.
For Saveto Copper Tape, the typical shielding effectiveness (far field) is in the range of 75dB to 85dB (30 MHz to 1 GHz).