What is the generally accepted number of syllables to count when taking a word count sample of a child’s speech for the %SS? Is this based on research or experience?

If taking a %SS measure, a sample of about 300 syllables is suggested. However, there are occasions where the clinician might decide that a longer sample is warranted if they do not think that they have a valid measure. On the other hand, if it takes too long to gather a 300 syllable sample e.g. for a quiet child, the clinician might choose to rely more on severity ratings or recordings from home.

LPTC - AN INTRODUCTION