Bias in the DRW decorrelation timescale caused by the experiment length. As contours we present two sets of 10 000 AGN light curve realizations modeled as DRW, SDSS (60 epochs; left) and OGLE-III (445 epochs; right), with SF_∞ = 0.20 mag and a wide range of input timescales, where r ≈ 17 mag and I ≈ 18 mag, respectively. AGN light curves that are long compared to the true decorrelation timescale, at least ten times longer (ρ_inp < 0.1), are the best source of the correctly measured decorrelation timescales (the “biased” region). Light curves that are short and comparable to the decorrelation timescale (ρ_inp ≈ 1) will not yield reliable decorrelation timescales and they are unconstrained (the “unconstrained” region). The typical measured timescale in the “unconstrained” region is ~20–30% of the experiment span, independent of the input (true) value. The border between the biased and unconstrained regions is ρ_inp = 0.1. Location of the border line is independent of the input amplitude SF_∞ and the source magnitude. While not shown, the typical uncertainty on ρ_out is half of the outer contours.