I believe they are permanent. There are no rules in the Temp HP section of the book that says they fade over time, just that they can't be restored (you could probably take this to mean new Cold damage effects would not stack more Temp HP on the golem), and are subtracted from first, before affecting regular HP.
I'm used to seeing them last for one hour. I don't think I'd expect to see a wizard cast maximized empowered polar ray on his golem and have it last for five years, upon which adventurers break into his tower for the first time.
Yet I'm not finding anything in the CRB stating that they all disappear. The description of hit points on page 12 says that some spells and effects may grant temporary hit points which disappear after a specific duration, but then, that isn't also necessarily limiting all temporary hit points to expiring.
Edit: Correction. The temporary hit points from Virtue doesn't disappear, but it does have a duration, so the temporary hit point leaves with the spell.
What Rynjin said: temporary hit points remain until something gets rid of them, either damage or the effect granting them ending. Looking at the ice golem, the effect that grants them is the Immunity To Magic, which is an extraordinary ability, so the only thing that will remove them is damage.