@Markus - There's a few factors that you'll need to consider before you can determine the life expectancy of your boats manifold system. First, of course it'll depend on the type of boat you have and secondly how you plan to use your boat.
And the third factor is an important one that most people tend to overlook and that is the type of water they plan to put their boat on.
If it's freshwater, such as rivers and lakes, and used mainly for fishing then your probably looking at getting a good fifteen to twenty years out of your manifold. If you're planning some heavy usage in saltwater, then you're looking at only three years, five at the most.