There is no minimum or maximum for a project completion party. The scale must fit what the project demanded. A big celebration for a small project feels over the top. A small drinks reception after a two-year mega project feels inadequate.
Guideline based on project duration and intensity: Sprint or quarterly project (4–12 weeks): retrospective drinks or team dinner, 2–4 hours. Large annual project (6–12 months): festive evening with programme, entertainment and dinner. Multi-year project (1–5 years): full event with everyone involved, including external partners and client contacts.
Remember: the size of the celebration signals how big the project was for you. Be consistent in that signal.