Statutory application periods vary by local council and by event category. As a guideline:
Small event (private, no traffic disruption): 8–12 weeks in advance. Medium event (public, 50–1,000 attendees): 12–16 weeks. Large or high-risk event (1,000+ attendees, multiple permits): 3–6 months.
But: the formal deadline is the minimum. In practice, council officers are busy and the review process takes longer than planned. Always submit earlier than the minimum deadline requires.
Also note: the event permit is one permit. Sound and alcohol exemptions are applied for separately and in parallel. Align the timing so you don't end up with a gap.