I stopped mining altogether. It's simply not worth it unless you want a bunch of iron equipment or have transmutation. By the time you find a mine that has what you want, you'll be selling gear for the ingots faster than it can respawn.
The only real point is for smithing purposes. I've read that smithing will let you get better armor than anything you can loot or purchase, and certainly you can improve armor and weapons with smithing, but it requires ingots (and leather) to do so. Given the steep prices merchants charge on metal ingots (even lumps of ore aren't very cheap to purchase) it's good to save some $$$ by mining the stuff yourself.
But a word of warning: Smithing is not a really good way to make money, and you can in fact lose your shirt if you don't pay attention to the money. http://www.uesp.net/wiki/Skyrim:Smithing; most especially, http://www.uesp.net/wiki/Skyrim:Smithing#Skill_Usage Long story short, if you buy the raw materials rather than hunt and mine for them yourself, many items are worth less than their constituent raw materials. Add in the merchants' markup when selling ingots and leather and their markdown when buying the finished product, and you really have to choose the right items to smith in order to show a profit.
Also, merchants' supply of the raw materials is rather limited. They aren't gonna be sitting around with 50 ingots of refined moonstone for you to go forging Elven armor.