Yes that is also true, if you building a website just for the money without having any interest in that subject at all then you will get bored easy, but if you build your sites because you like the subject that you are building them around you will not get bored because you know the subject well and will always have new content to be adding etc etc Y dont many people build sites for fun, my site is there for a reason and it certainly aint for the money, and i know i certainly have loads of fun building it etc.
Like a lot of things in life, it depends on your circumstances. For me, as a software developer, making money with Adsense is like being a fish in water. If it takes 6 months to prepare a website, before making dime one, it makes no difference to me. For others, the time and energy required to get good at this is simply unacceptable. They get frustrated and bored too easily. They are not self-motivated and they are looking for the fast buck. If you treat this like a business, my experience is you will DEFINITELY see very satisfactory returns within in a year.