The Sprint Planning is an event that occurs at the beginning of each Sprint, where the Scrum Team plans how to deliver a valuable product Increment. The Sprint Planning consists of two topics: What can be done this Sprint? and How will the chosen work get done? The outcome of the Sprint Planning is an agreed-upon Sprint Goal, a Sprint Backlog, and a plan for delivering the Increment.
The timebox for the Sprint Planning event is proportional to the length of the Sprint. For a one-month Sprint, the timebox is 8 hours. For shorter Sprints, the timebox is usually shorter. The Scrum Team may decide how much time to spend on each topic within the timebox, depending on their needs and preferences.
The timebox for the Sprint Planning event ensures that the Scrum Team does not spend too much or too little time on planning their work for the Sprint. The timebox also ensures that the Scrum Team has enough clarity and alignment on what and how they want to achieve during the Sprint.
Reference:
Scrum Guide: https://www.scrumguides.org/scrum-guide.html
Sprint Planning: https://www.scrum.org/resources/what-is-sprint-planning