What constitutes an easy ball varies with each player. For almost anyone, a slow ball 2-4 feet above the net and less than 8 feet away should be worth attacking. The more you poach, the better you'll get at it, and you'll be able to go after lower, faster, and more distant balls.
For opportunistic poaches, start moving as soon as you see where the ball is going, which should be shortly after it leaves your opponent's racquet. For planned poaches, start moving as soon as the opponent starts her swing. Unless your partner will be crossing over to your side because she knows that you plan to poach, you can start to poach, then change your mind if the return is better than you expected.