This is simply incredibly hard to answer, to be honest. Though, I think a fair amount of people would agree that a large amount of time is spent on simply creating the assets. After all, they are what’s going to be seen by the user. It will truly vary from game to game, and on which platform(s) you are targeting.
Not entirely sure what you mean here, I’m afraid. There is a plethora of resources, to be found online, which will allow you to learn almost every aspect of the process. You’d simply need the time and drive to put them into use. It’s certainly possible to learn everything - though. Mastering everything is slightly harder. After all, we each have our strong and weak points.
Entirely up to you. At the very least, you could use marketplace assets to quickly prototype your project and replace them with final assets at a later stage.