Rusting Grasp Q

[Galfridus]

The description for Rusting Grasp (PHB) says "see text" for duration. The text has three "modes" of use: against a metal object, against armor, and against a ferrous creature. For the last case, the duration is stated as 1 round per level, but for the other two no duration is stated (and I assume it's different, since if it were the same for all of them they could have just put "1 round / level" at the top). Anyone know what the duration is in these cases?

 

[shilsen]

The spell description doesn't spell it out explicitly, but it seems to me that for attacks against armor or a metal object it is discharged after a single successful use (like most touch spells).