The timeline for making a toothbrush is about one afternoon 
What you really want is a sustainable business, based on an original idea. And then comes the toothbrush. How is it going to differentiate from the already existing ones? How are you going to get your brand to be recognized? How is it going to appeal to a specific population? You need a business plan first, even if it is only 5 pages. Once you have an idea based on initial user research/brainstorming/mindmapping etc you can start materializing it, which will infuse your creative thinking as well. I posted an idea to get you going. If you want to design it yourself I suggest to read up on the history of industrial design so you can place your design in a socio-cultural context, on materials and on form/3D design. Burdek’s ‘History, Theory and Practice of Product Design’ gives a good introduction of the field and Rob Thompson’s books are great for getting an overview of production and prototyping methods.
