Knowing when to trademark is important, but not as important as the search. The most vital step is to do a comprehensive US trademark search before spending any money on marketing and merchandising. You do not want to waste time and money on a mark that is strikingly similar to another product or service.
If that is not an issue, I recommend that you trademark your startup shortly after you incorporate. I suggest this for two reasons:
First, if you’ve done a comprehensive search (which you’ll do during the filing process), you can find out if you’re actually infringing on another company’s mark. This will help to avoid significant time and legal fees in the future.
Second, it is possible to claim ownership of the mark without formal registration. However, without registering with the US Patent and Trademark Office (USPTO), your rights are limited to the geographic region where your mark is used. So, by formally filing, you can have protection across the US.
There is an article on my company’s blog that discusses this issue in more depth. You can check it out here: How soon should I apply for trademark registration?. I really do recommend working with an attorney familiar with the trademark process. If you’d like assistance being matched with a skilled trademark attorney, feel free to reach out to my company LawTrades.