I've waited over 10 days, the mine is still empty.

I hink its an ingame month.

So at least 30 days?

Just go to belethor in whiterun, he sells them 400 a pop.

That's what I do, but it would be nice to have the entire mine respawn, which will save me some dough.

I heard 30 in game days. I however had to save, quit, and reload the game for the ore to be mined again. (as in I saw the rock change from blank to having ore, I was unable to mine it until restarting. )