It's best to register your trademark before you start using it, as it's best to register it as soon as possible. Since trademark registration can be done later out, it is necessary to avoid the risk of trademark registration by others first.
However, it is also a fact that if the trademark is not recognized by customers as its own brand, there is little need to protect it even by registering the trademark. You may not need to register a trademark for a limited-time product name that you do not plan to continue selling.
The best time is "the stage when the trademark is recognized by the customers as identifying the company's product and there is a possibility that the trademark will be imitated by other companies". For example, when it is introduced on a TV or other massmedia, you should apply trademark registration before it airs, at the latest.