This is music. Music is art. Art is structureless. In other words: YOU decide what structure you'll use. A 16bar intro? a 32bar intro? a 48bar intro? It's up to you. Just keep in mind that human ears are most accustumed with 2X bar sections, so for a more "ear friendly" structure, the perfect spot is to use 4 bar, 8 bar, 16 bar, 32 bar, 64 bar and so on.
As Mania said, listen to your favorite artists songs, and count the bars, write it down and there you go.