In general, it should take about 10 to 20 minutes for a person to drift off. But if one perceives that it is taking too long—whether it takes 20 minutes or an hour—that can spur anxiety about sleep that negatively affects sleep quality. Falling asleep as soon as one's head hits the pillow is not proof that one is a good sleeper; it's more likely an indication that an individual is sleep-deprived.
For more on recognizing signs of disordered sleep, see Overcoming Insomnia or Sleep Disorders.