  In any flux-density limited sample of blazars, the distribution of the
timescale modulation factor $\Delta t'/\Delta t$, which quantifies the change
in observed timescales compared to the rest-frame ones due to redshift and
relativistic compression follows an exponential distribution with a mean
depending on the flux limit of the sample. In this work we produce the
mathematical formalism that allows us to use this information in order to
uncover the underlining rest-frame probability density function of measurable
timescales of blazar jets. We extensively test our proposed methodology using a
simulated FSRQ population with a 1.5 Jy flux-density limit in the simple case
(where all blazars share the same intrinsic timescale), in order to identify
limits of applicability and potential biases due to observational systematics
and sample selection. We find that for monitoring with time intervals between
observations longer than $\sim$30\% of the intrinsic timescale under
investigation the method loses its ability to produce robust results. For time
intervals of $\sim$3\% of the intrinsic timescale the error of the method is as
low as 1\% in recovering the intrinsic rest-frame timescale. We applied our
method to rotations of the optical polarization angle of blazars observed by
RoboPol. We found that the intrinsic timescales of the longest-duration
rotation event in each blazar follows a narrow distribution, well-described by
a normal distribution with mean 87 days and standard deviation 5 days. We
discuss possible interpretations of this result.
