As far as I know there is no air time in question. It depends on what you purchade. Example; if you purchase album, then you would have 8-12 songs which you could listen too. If you buy a single, then one song is what you would have. Singles normally cost between .99 to 1.29+. Album prices vary.
This means for a $25.00 iTunes card you may purchase up to $25.00.