What is the best time to patent our product?

Actually, your actual product may not be a valid patent as a whole. Yes, if you could narrow to the core invention of your product, you might find that, just one part of your product might be an invention.

Technically, It is better not to club the patent and the product together. Because, if you find that there is a inventive step, and it is non-obviousness, and if your whole concept sounds positive and there is a space for market potential for developing a product, then it is better to file the patent irrespective of the product development.

Economically and logically, if the product research is at developing stage, and your research is protected by trade secret strategies, then continue product development without a patent. Specially, in the US Market, creating a patent asset is a costly process. Hence, consider patenting at very positive stage of development and market potential.