The best method is to keep track from the beginning of EV training - count each battle (modified by the amount it gives, items, etc) and keep a running total as you go. That way, you know when one stat is done and you can move onto another.
If you haven't kept track, there are two options:
You could use EV reducing berries from the Dream World to reset the EVs and start over. This might seem like taking two steps forward and one step back, as you'll have to re-earn what you already put into it, but at least you'll know when you hit your goal.
The other method is to train it in whatever other stat(s) you want to focus on (keeping track this time). Once done (meaning completely full in the other areas you want to work on), return to talk to the woman in Opelucid again. If you reached the max for Defense, she'll tell you that you can't gain more. If you have room, she'll let you know that there's still space.