When should I start pursuing sponsorship renewals and new sponsors?

Start a year to a year and a half in advance of when you need the sponsorship. If you're waiting until December for a January season, those budgets are already allocated. You need to be in the conversation before budget planning happens so you get included in that budget. This requires knowing whether your target sponsor is on a calendar year or fiscal year (Japanese companies like Honda operate April 1st fiscal years). Advanced planning and understanding their budget cycles is essential for being considered.