There is no defined time. A fixation can be something rather swift that only lasts for a fraction of a second.
If Gaze Viewer is used with Tobii Gaze Interaction Software, which is the control software used for e.g. the I-Series, C-Series and PCEye Go, we use a fixation filter based on angular velocity of the pupil. If Gaze Viewer is used with Gaze Point, e.g. PCEye Explore, we use fixations provided by the underlying EyeX frame work.