Power spectral density (PSD) for DRW. We mark the frequencies τ^-1, (3τ)^-1, and (10τ)^-1. The experiment length shorter than ~ 10τ is insufficient to measure τ because such light curves simply do not (for ν>τ^-1) or weakly (for ν> (10τ)^-1) probe the white noise part (PSD ∝ ν⁰) of the spectrum. The inset presents the histogram of the SDSS Stripe 82 experiment lengths (hence the longest timescales probed) divided by (1 + z) for ~9000 SDSS AGNs and 500 I< 19 mag 20-years-long AGN light curves from combined phases of OGLE. If the true decorrelation timescale is one year then the SDSS S82 data do not probe the white noise part of PSD, so cannot constrain the decorrelation timescales.