A method is proposed to experimentally determine the intrinsic time scale or a decay rate of turbulent fluctuations. The method is based on the assumption that the Breit-Wigner spectrum model with a non-Gaussian frequency broadening is valid in the data analysis. The decay rate estimate is applied to the multi-spacecraft magnetic field data in interplanetary space, yielding that the result that the decay rate on spatial scales of about 1000 km (about 10 times larger than the ion inertial length) is higher than the theoretical predictions from the random sweeping time scale of the eddy turnover time. The faster decay of fluctuation components in interplanetary space is interpreted as a realization of plasma physical (and not fluid mechanical) process.