You can start to define your company culture at any time in a company's life cycle. Your culture exists, whether you like it or not and the longer you wait, the harder it gets. If you haven't defined it yet your company's culture is developing invisibly and unconsciously - by default, not by design. Bad habits and practices are creeping in as the company makes progress and the team grows. Reversing or undoing these habits and practices is hard to do because they have become ingrained into the "way we work around here", and on the surface they appear to be working.
It is possible but it takes a lot of management time and effort to reverse a malformed culture before being able to move forward. So it's best to start early.
There are 2 key stages of a company's development, where the implementation of company culture is most effective and efficient:
1. When you found the business
Instead of choosing to focus on only building the product, you focus on building the product and defining the culture at the same time. Granted, it takes more effort at a time when there is less time and resources are scarce, but you are investing in the foundations of the business and the ROI is real. Take hiring. If by defining your company culture you eliminate the hiring mistakes you would have made, without being able to evaluate a candidate's cultural match with your company, the time and money saved will be significant.
2. Before you scale the business
Once the product market fit is in place and the business model works, the company will look to raise funding and start to scale. This situation normally results in companies being desperate to hire bodies for the roles that are needed, and if the culture has not been defined it's almost impossible to hire the right candidate culture match consistently. As new employees are brought on board and the founders become spread thin, they are no longer able to be the effective cultural glue for the company that they were and the original culture starts to dilute. This is where a lot of scaling startups fail.
So when will you start to define your company culture?