The rule of thumb: for an event of a hundred people or more, you start at least three months in advance. For large events with external speakers, complex technology or an international dimension, six months is realistic. For small team events or department outings with twenty to fifty people, four to six weeks is enough. Provided the venue is available.
Always start by locking in the date and the venue. Those two elements are the foundation of every event. Everything else (programme, catering, invitations, communication) you build afterwards. Without a confirmed date and venue you can't plan anything, because everything depends on those two anchor points.