Having identified the 4 elements of your brand identity: the brand’s 3 P’s of marketing (Performance, Pedigree and Persona), the style, the price positioning and the price pyramid, you can now give a detailed brief to the designer that you chose to hire (see my recommendations on what to look for).
Realistically, you should expect any development to last at least 3 months for prototyping and 6 month for mass production. The duration of the design phase that precededes depends on your iterations with the designer and that person’s familiarity with the field (hence the importance of choosing a designer who is familiar with watches).
Let’s see into detail the various phases, and the cues that let you know it is time to move from one to the next: Concept and Design Detailing.
This is the hardest part. The work needs to take into account the four disciplines of Marketing, Art, Engineering and Artisanry to create something that balances function, form, constraints and finishing (see Materials and Processes).
It needs to be cost-efficient and consistent with its assigned position in your brand’s price pyramid, but it also needs to have enough visual and structural details to create a positive reaction in the customer.
I personally split the work between parametric 3D modelling for the case, buckle and visual merchandising (because the manufacturing process of these components is tridimensional), and 2D artwork for the dial, hands and strap (because they are essentially manufactured out of flat material: brass sheets and soft/textile material). If your project requires a polymer strap, then it would be suitable to model it in 3D as well.