The original Column of Ice spell wasn't home brewed, which seems to be what you're suggesting. It's from Frostburn.
Oh. Well, then it was officially messed up IMO. This sort of poorly thought out poorly edited crap is exactly why I never bought any 3.5 books. They were being churned out without any oversight, any editing, or any discussion (or at least none that was apparent) just as fast as someone could write up the content. Most were worse that useless. They were actually destructive to the game. It was like the company had decided to actively undermine their own game system.
I suspect that it has the Cold descriptor because you're conjuring ice.
I figure that is a safe bet, but it seems like a failure to understate the precepts of the descriptor system. You may be conjuring ice but within the descriptor system, cold doesn't necessarily have a lot to do with ice - as the spell description itself proves. You may have conjured ice, but per the spells description the ice is not cold! The Cold descriptor is appended not to spells related to ice, but to spell that cause cold energy damage - which this spell does not do.
A safer descriptor would be the element actually conjured - in this case paraelemental Ice. I think the confusion comes from Fire, which is both an element and an energy type. But Ice is not an energy type, and Cold is not an element. If this spell in fact created 'Cold', then it is in the wrong school - it should be evocation rather than conjuration (creation). But it doesn't create cold, it only creates ice.
The effect is "permanent", and gives no guidelines for melting time, or if there even is one, so I'd agree that it's poorly formed, or at least poorly written.
But if it's messed up, it's at least *officially* messed up.
I'm not sure you need guidelines for how fast this thing melts as part of a spell, but it probably should say something like. "This is normal non-magical ice, and it will melt or sublimate at a typical rate for the surrounding environment." In a small dark space, that may in fact be weeks or months. Of course, the text doesn't say that and in fact applies something of the opposite - this is explicitly magical ice. It's not cold and it doesn't melt. It's just there.
Note that text like that only reinforces just how thoughtless (if understandable) it is to give this spell a Cold descriptor. However, I think you are all getting confused over the Energy Substitution feat which substitutes ENERGY and not element. If you use Energy Substitution to make this a Fire or Electricity spell, you still get ice. For example, Energy Substituted to fire, this is what you get:
"A column of ice rises from the ground, lifting any object or creature (including you) standing in the area into the air.
Creatures making a Reflex saving throw can choose to avoid the column. Moving or fighting atop the ice column requires a DC 10 Balance check. Those who fail fall prone and must immediately succeed on a DC 12 Reflex saving throw or slip off the column, taking commensurate falling damage. Creatures atop the column as it rises may be smashed against the ceiling or other overhead obstructions, which deals 4d6 points of damage.
Magical Ice Column: 10 feet thick; hardness 16; hp 160; break DC 90; Climb DC 30.
Arcane Material Component: A 2-inch rod of ice."
The spell doesn't actually change at all. Energy Substitution just replaces all instances of the word 'Cold' with the word 'Fire' (for example). Since this spell isn't actually Cold (despite the misleading and erroneous descriptor), it does exactly nothing to substitute it. It creates a column of ice period. Nothing in feat says that you get a different element when you change the energy type. That's an assumption that is being made. Energy types are related to elements in a semi-complex manner, and for a typical transient energy releasing spell that Energy Substution actually works on, you can change the flavor of what is manifested without harm. But in this case, that certainly doesn't happen. In this case, changing the descriptor only changes how hard the spell might be to cast in the local environment (assuming you have the component) - for example, you could use Energy Substitution on the spell to easily conjure ice on the Elemental Plane of Fire which might otherwise be difficult. (And once again, this shows why the descriptor should be Water or Ice.)
Further note that I'd generally never approve a spell of this sort because creation spells of unlimited duration have huge unintended economic impacts on your campaign.
