  We study the intrinsic, disorder-induced decoherence of an isolated quantum
system under its own dynamics. Specifically, we investigate the characteristic
time scale (i.e., the decoherence time) associated with an interacting
many-body system losing the memory of its initial state. To characterize the
erasure of the initial state memory, we define a time scale, the intrinsic
decoherence time, by thresholding the gradual decay of the disorder-averaged
return probability. We demonstrate the system-size independence of the
intrinsic decoherence time in different models, and we study its dependence on
the disorder strength. We find that the intrinsic decoherence time increases
monotonically as the disorder strength increases in accordance with the
relaxation of locally measurable quantities. We investigate several interacting
spin (e.g., Ising and Heisenberg) and fermion (e.g., Anderson and
Aubry-Andr\'e) models to obtain the intrinsic decoherence time as a function of
disorder and interaction strength.
