Yes, this has been serious problem from the beginning, and it got much worse the day practice was restricted to the skill ranges we have for each crafting process (see bug tracker link above). As Gilrond stated, this does not seem to be a simple matter of tuning a few numbers, but a fundamental flaw in the current set of rules (fiddling around with those ranges will probably affect other things). On the other side, practicing certain other skills does not only take orders of magnitude less time, but really also implies much less human effort (mob auto-bashing while having a chat, training magic while earning some money etc.)
I think the natural solution to this problem is to penalize endlessly repetitive training sessions. The amount of practice awarded should not only depend on the difficulty or time requirements of an action, but rather on its learning value for the character to experience something new and hone his skills. Okay, this sounds abstract and hopelessly difficult to implement. Well, you could think of a finite history of recent practice activities for each character. Beating up the same type of NPC, spamming one spell all over or repeating the very same crafting process would fill up the list with patterns of repeated action IDs. This could be interpreted as an effect of exhaustion or boredom that will drastically lower the gain of any following practice, unless you clean up your history by doing other things, which will discard the older entries. I think with such a system, the complex and tedious crafting patterns will have a natural advantage, because in order to produce something reasonable, you will use many different processes. The times of endless handle hammering would be over. Limiting skill ranges for practice could be dropped entirely and the gap between casual practice and stubborn grinding could be narrowed down a little, too.