The honest answer is: earlier than you think, and for less money than you fear. A founding team does not need a law firm on retainer in month one. It needs three or four decisions made correctly before they harden into habits that are expensive to undo.
The founder agreement comes first
Almost every dispute I see between co-founders traces back to a conversation that was never written down. Who owns what percentage. What happens if one founder leaves after six months. Whether unvested equity returns to the company. These are uncomfortable questions to raise between friends starting a company together, which is exactly why they need to be raised early, while the relationship is still easy.
A founder agreement is not a sign of distrust. It is what makes trust durable once the company gets hard.
Contracts you sign before you understand them
Early customers, first suppliers, an office lease — each comes with a contract that someone on the other side has already had reviewed by their own lawyer. Signing quickly to close a deal feels like momentum. Often it is simply accepting terms written entirely in someone else's interest.
Before you take investment
By the time a term sheet arrives, the important decisions have usually already been made informally, in conversations the lawyers never see. Understanding what a term sheet actually gives away — control, economics, future flexibility — is worth doing before that conversation, not during it.
What this actually costs
A short, focused engagement at the right moments in a company's life costs far less than untangling a governance dispute two years later, and considerably less than the cost of a deal that falls apart in due diligence because the basics were never in order.