I find that 5 to 8% is the right range for my preferences. I'd say that over 10% and lactose gets obvious and a bit cloying.
For me, the effect of lactose (sweetness and creaminess) is amplified or limited by the proteins and dextrine left in the beer. Clearly, dextrine will add sweetness of its own, which may not be what you want. I find that a little bit of oats adds the right amount of silkiness to amplify a modest amount of lactose.
Yeast can ferment lactose, but it's a very slow process. it only matters if you're trying to store the beer for while (several months) or use a high amount.