My son goes for trumpet. They checked his mouth and teeth, measured the length of his fingers, asked him to keep closing and opening his fingers quickly (both hands at the same time) for a minute. They asked if he knows what instrument he has chosen and also asked about his music background. It took about 15 mins.
I heard from a boy (chosen violin) that they asked him if there's a video game and a violin in front of him, what will he choose.
I have observed that mostly everybody took 15 mns.