I always heard it was half your age PLUS seven years. Even by his own math, half of 42 is 21 and subtracting seven from 21 is... still statuatory rape.
@JaeMargal If they're struggling, it takes as long as it takes. If they're hiding the struggle, then it usually takes longer. If they're a narcissist, no time at all. And if they're a third-culture kid, just tell them it's like they moved and they'll get over it.