How long should it take to develop a product?

First of all, there is no single answer.  It depends on the industry (whether it is regulated / whether there are compliance requirements), product type (hardware, software, bio/pharma, construction/building based, other), and complexity (for example, consumer electronics products have a huge range in complexity from a powerpoint clicker to toys to computers and mobile devices to small kitchen appliances and more).

That said, there are some order of magnitude references that can be useful. As of 2015, here are some general guidelines for "what's reasonable".

• Mobile apps – no science: 0-3 months 
• Web apps – no science: 0-3 months
• Web apps, big data science: 3-6 months+
• Embedded software releases (e.g. firmware running on a car): 6-9 months after the hardware is stable
• Small scale mass produced consumer electronics products : 9-12 months (e.g. a new tablet)
• Complex industrial equipment with mechanisms: 12-36 months (e.g. Tesla S)
• New drug: 12-18 years