Good question, it depend on lots of things really. Satellites can vary in size massively, from very small “CubeSats” that weight a couple of kilograms right up to massive communication satellites that weigh many tonnes.
The time they take to build varies massively too. A simple CubeSat mission could be put together in a couple of months, whereas a large science mission (such as a Mars rover) could easily take 10+ years and require hundreds and hundreds of engineers. As an example, the camera I’m currently working on for the International Space Station will take around 18 months – and this is considered to be very quick! If I had to generalise – a typical satellite might be 5 years.