The user must specify the exposure time; that is, the time during which photons are collected to record an image (minimum allowed value is 0.001 s, maximum allowed value is 60 s).
The user should always verify that the percentage of the full well capacity of a pixel filled by the highest peak of the PSF is below 100%.
Actually, the maximum suggested exposure time is the one where the PSF peak fills 85% of the full well capacity (to provide enough margin to clearly avoid pixel saturation). It is also recommended to select an exposure time close to this maximum suggested exposure time (or 60s if the 85% cannot be achieved), to keep the instrumental noise to a minimum.
On the other end, the minimum suggested exposure time is the one corresponding to the PSF peak filling 10% of the full well capacity.
The user is responsible for selecting the correct exposure time and may have reasons not to follow the guidelines given above. For example, the user might be interested in shorter exposure times than recommended to have a faster cadence of images or imagettes (see Observers Manual, Table 2).