The purpose of this study was to determine the optimal amount of time a supervisor needed to observe a teacher to obtain a representative sample of his teaching behaviors while Direct Instruction was provided. These data were gathered across two Direct Instruction programs—Spelling and Reading Mastery—presented by a teacher with over 10 years of Direct Instruction experience. A Direct Instruction observation instrument and rating form were used to gather and summarize these data across 3-, 6-, 9-, 12-, 15-, 18-, 21-, and 24-min observation sessions. Results indicated that although there were minor variations in the percentages of teacher behaviors and scores received based on these percentages, the length of observation did not greatly affect the grade given to the teacher. However, the least amount of time required to obtain a representative sample of the teacher’s instructional behaviors was 12 min. Implications for supervising preservice teacher trainees and making data-based decisions on trainee performance are discussed.