We present the first experimental determination of the electric-dipole forbidden (3s3p)3P2¿(3s2)1S0 (M2) transition rate in 24Mg and compare to state-of-the-art theoretical predictions. Our measurement exploits a magnetic trap isolating the sample from perturbations and a magneto-optical trap as an amplifier converting each 3P2¿1S0 decay event into millions of photons readily detected. The transition rate is determined to be (4.87±0.3)×10-4¿¿s-1 corresponding to a 3P2 lifetime of 2050-110+140 sec. This value is in agreement with recent theoretical predictions, and to our knowledge the longest lifetime ever determined in a laboratory environment.