Map of C IV response timescales on the plane of the ionization parameter and the gas density. The dashed lines represent contours that correspond to observation intervals ΔT of 1, 10, and 100 days. The dashed gold line marks one day, which is the typical minimum time interval of SDSS observations. Below each contour, the observation interval is shorter than the response timescale, indicating a nonresponsive region in which no ion concentration changes can be detected.