The first time you need to get a 409A valuation is before you issue your first common stock options for employees. Founder shares are typically issued at incorporation and do not require a 409a valuation. If you are not planning to issue common stock options, then you can wait on issuing a 409A valuation. After your first 409A valuation, you need to update your 409A valuation at least once every 12 months, and whenever a material event has occurred (e.g., a new fundraising round.)
Material event: something that affects the value of the company. A standard "material event" is closing an equity fundraising round or SAFE round. Other "material events" can include: acquisitions, secondary sales of common stock, significant changes to your company's financial outlook, upcoming IPO
Disclaimer: We are not lawyers, and this is not legal advice. Although we try to make sure our information is accurate and useful, please consult a lawyer if you want legal advice.