Time required to build an entirely custom website: Weeks-to-months, depending on the complexity of the project and your coding skills. One web-design blog estimates that it takes approximately 14 weeks.
There's no official average, as no two plugins are the same. If your plugin is small and all the code is correct, it should be approved within fourteen days. If your plugin has any code issues, it will take as long as it takes for you to correct the issues.