Hi Megan,
It depends on what eye tracker and software you are using. The Tobii REX has a fixed frame rate at 30 Hz and always gives you data. It uses different error codes if the data is not considered valid because of for example a closed/not detected eye. The Tobii EyeX Controller does not have a fixed frame rate. If you use the EyeX Controller together with the EyeX SDK to retrieve gaze data, you would normally have a frame rate around 50 Hz, but no gaze data at all if the user’s eyes are closed/not detected. The average frame rate of the EyeX data streams might be changed in any future EyeX Engine release.