They already have things that affect where trees can be placed in relation to other objects, since trees don't grow unrealistically close together. And they already have something that references rocks, since there are no trees that grow through rocks. Thus the framework already exists.
I'd be somewhat surprise if it took more than half an hour.
I've done some basic programming in my time, but on a very small and limited scale. Even on that small scale,
no major change to an algorithm/calculation takes a half hour. It doesn't matter if the framework is already in place or not. The reason is that it is not just about coding that change, it's about doing all the testing required to make sure that the new code does what you want it to do and doesn't interfere with anything else. In a sandbox world like Skyrim, that testing is going to take many half hours to make sure it works right (otherwise what's the point?).
Our ideas on how long it would take, though, are all purely speculation. Personally, I would be dumbfounded if it took less than a half hour from soup to nuts. But again, I have no idea. What it really comes down to is how important Bethesda thinks this issue is. Game development is a zero-sum game--any time spent dealing with one issue is time that is not spent dealing with another issue. Issues need to be prioritized, and if something is low enough on the list of priorities it is not going to be addressed before release.
Apparently this issue bothers you somewhat, and since you are convinced that fixing it could be accomplished over a lunch break, I can understand why you would expect the change to be made. I'm not starting from the same set of assumptions, though, so I reach a different conclusion. Who knows what is actually going on inside Bethesda?
[Edit: in other words, maybe it's an important issue to them, maybe it isn't--and maybe they've already fixed it!]