From what I remember, it's a lot of work in spraying the different stages. When I used to work at a local car dealership (about 12 or 13 years ago), one of the guys in the body shop was repainting a boat of his with this type of stuff. From what I remember, it was about a 5 stage process, or at least the effect he was going for took about 5 stages. First he laid down a dark base colour. After that he laid down the first colour of the metal fleck paint. After that he sprayed again with a lighter coloured transulcent version of the first base layer. The fourth layer was a different coloured metal fleck again. After all this, he sprayed a few layers of clear coat.
I guess it depends how much depth you want to create in the paint/gelcoat. The more depth you want, the more layers you'd have to put down.