Images of the full well depth over the two UVIS detectors are shown below (Fig 5 from ISR 2010-10). The WFC3 CCDs show about 10% variation over each CCD ranging from 63-71 K electrons on UVIS1 and 67-72 K electrons on UVIS2. The onset of saturation is taken to be the level at which charge spilling along columns first starts and provides the definition of full well depth.
ISR 2010-10 provides a detailed description of how the full well depth was evaluated. It also provides simple equations that allow photometric data well beyond saturation to be restored to excellent linear response. These equations contain a dependence on the full well depth local to the position of any object to be corrected.