Maxing armor skills vs rats feels a bit odd indeed. I have not made it so high in armor so I don't know how it is on higher levels. However fighting trepor armor will not raise above 15. It felt a bit low but instead of complaining about that, I thought that maybe it's possible to have some kind of efficency coefficent. Like fighting rats, up to level 10, you gain 100% armor practice. After that it gets lower up to level 20 for example where you will gain 0%. Numbers are just examples

 If you feel that a mob is too though to practice on, you can go back and fight something else, though with slower gain. This would add more flexibility as you can fight more mobs for training, higher lvl for faster or lower lvl for slower gain but it would still be a limit so you can't max out at low level mobs. Any thoughts about this?

EDIT: For example, at 15 Heavy armor I can't gain anything vs trepors or tefusanglings. My option is to move on to gladiators but they will damage me for over 30hp with each hit which means there is not much gain before one has to stock up with more potions. Judging from how much the bar has moved it will be an hour or two just to get from 15 to 16. I can't imagine how much time one has to spend at higher levels since one also has to fight more powerful mobs.