Extended testing time is a common accommodation given to students with disabilities. However, little is known about whether students who are eligible for the accommodation actually use it, and with what effect. The present study used process data available from the digitally delivered 2017 National Assessment of Educational Progress (NAEP) mathematics test to explore these topics. We found that although the majority of students with special education plans were eligible for extended time accommodations, most of those eligible students did not actually take any additional time at all to complete the test. We describe differences in eligibility for extended time and use of extended time that were identified according to gender, race/ethnicity, disability type, family income level, English Learner status, perceived importance of the test, and use of the text-to-speech (TTS) accessibility feature. Use of extended time (among eligible students) was associated with higher performance. Implications for practice and future research are discussed.