The observation of initial time dynamics of self-trapping in photorefractive media indicates that optical spatial solitons supported by intense cumulative nonlinearities manifest temporally nonlocal signatures in the form of stretched exponential behavior. This general result, supported also by numerical predictions, is triggered by wave shaping in a time-constant buildup map, a consequence of the spatially resolved inertial response intrinsic to the geometrical transition from a diffracting to a self-focused beam, inherent to soliton appearance.