The key property is hardness. For uniform materials (like cogs), hardness directly affects wear resistance. The harder the metal the longer it will last.
Some digging around wikipedia suggests that typical Brinell hardness values are:
* pure aluminium 15HB,
* 6061-T6 aluminium (heat treated) 95 HB
* mild steel 120HB,
* 4130 CroMo steel 183-217 HB (90-96 Rockwell)
It seems likely that most aluminium cogs will be 6061 or closely related alloys, so you're looking at the steel being about twice as hard as the aluminium ones. So it should last about twice as long.
Since people generally replace the entire cassette because the smallest cog is worn, by using an aftermarket one you're prolonging the life of the rest of the cassette by only changing it when the second smallest cog is worn.
Sorry for all the links, I couldn't find a single page with all the info on it.