I trim my own so usually they get filing at around the 3-4 week mark, and a full trim between 6-8 weeks; it all depends on the horse.
My mare starts mild flaring around 3 weeks, so she gets a good filing, to get rid of the flares, and fix any edges that may be 'scraggly'. She has all white feet, and when I got her she had terribly long, and chipped cracked feet; she's improved greatly with proper care.
The two younger horses I look after have really solid feet, and no flares, so they usually do fine with a strictly 6 week schedule.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."