You battle the Trainers w/the EV's you need? For 1 Pokemon, with a standard 252/252/8 EV spread, may take an hour, but more than 1 and heavier spreads...idk.
Sounds about right.
With PKRS and Vitamins you take out 16 Pokemon (15 to get 150 and 1 to get 2 EV's).
W/o PKRS and Vitamins, you have to kill almost twice that.
Naturally you also use the Power Items...but still.