It specifically says that you gain darkvision for 24 hours. This statement hasn't any bearing on the see invisibility portion, as they are two distinct sentences. There is no rule that states that all parts of a spell, invocation, or anything must have the same duration that I am aware of, so you'll need to find a quote to back that up. The "as see invisibility" part strongly suggests that you use the duration of see invisibility for this aspect of the ability, and there is nowhere else to find the duration of the effect.