The duration of a Gamma-Ray Burst (GRB) is often used to infer the progenitor system. Yet, the observed durations of the prompt emission from GRBs are influenced by the instrument sensitivity, energy range, background, and multiple other factors. For Swift/BAT, we identify three instrument parameters with the strongest influence on measured light curve duration: the number of enabled detectors, the incident angle relative to the detector plane, and the background level. Here we simulate synthetic FRED and real GRB light curves with varying instrument parameters to investigate the instrumental effects on measured durations of GRB prompt light curves. We find that the measured duration is often significantly shorter than the intrinsic duration and that the uncertainties of the measurements are often underestimated. The instrumental effects are particularly important for light curves with soft-dim tails, which are easily lost into background noise. In the case of GRB090510, a Swift/BAT observed burst with a short-hard spike with a duration of ~0.3 s and dim-tail which lasts until ~T0+5 s, we find the burst to have a measured T90 ~ 0.3 s in ~50% of our simulated Swift/BAT with varying instrument parameters.