The planning time needed for a farewell party depends on the scale. An informal drinks gathering for the immediate team can be arranged in a week. For a party with an external venue, catering and a video montage you need at least four to six weeks.
In any case, start as early as possible with the venue. Good restaurant spaces or event venues are quickly fully booked, even for smaller parties. And also reserve time with colleagues early for the personal contributions: texts, photos, video messages. They take more time to collect than you think.