I personally support a once a week changeover CHECK (not automatic, but the current chance of a vein change applied to all locations at once, once per week).
My research has indicated that, Dev statements to the contrary, all locations have a set percentage chance per swing of their colored ore coming out (if currently an iron location, this is only seen when you use a tool to get DC of Shadow from it), that DOES NOT CHANGE. The elf bonus is then applied to that (and in my testing produced little visable difference, unless a very low quality vein - the best veins consistantly produce 80-96% colored ore every time, to both races).
With BODs I get from 6 BOD runners (all colored BOD capable), I cannot mine enough DC ore to fill what DC BODs I get from those characters, without dedicating 12-20 hours of mining a week SPECIFICALLY for DC. I can get the all the other ores (via tools, or in the case of shadow, supplementing my mining with hunting orc brutes) in less time, in one trip for all of them.
I can see both sides of the argument, and frankly, both extremes are a bit weak. The idea of limitless resources of one type is bad, but so iis too random of change. In real life, many mines for metals, have other metals as "impurities", and depending on the value of the various metals, the market can shift, and what was a copper mine producing small amounts of silver & gold as the result of removing the impurities from the metal, can end up being a silver of gold mine primarily, earning extra income from selling the copper from the tailings.
Nothing "mystical" about it, as it happens in the real world - just not as quickly as in UO. 1 time a week technically translates into 2.5 months of 7 day in-game weeks, which is a little more realistic.