It's not unknowable; it's a cost. Because you can beat up bandits, get weapons, smelt weapons, get raw materials, refine materials, smith items, smelt items... there's very little waste in the system and you could advance smithing ridiculously quickly (ignoring the exploit of smithing items you don't have the skill for, which allows that anyway). The cost is time; if you want to grind major smithing sessions, there's a cost in that the world is moving on without you while you rest. Sometimes that's fine if you were wounded, or are trying to get your wife pregnant, or your companions are doing recruitment runs; but sometimes you're missing quests, opportunities to to intervene in a conflict, time you should be consolidating before the next declaration of war, etc.