The purpose of this study is to examine the effect of the duration of students' augmented reality (AR) experience on reading comprehension and cognitive load in reading activities carried out with an augmented reality storybook. The participants are 134 secondary school students aged between 11 and 12 attending a school in Antalya. They were divided into three groups. The same texts were designed with different AR durations producing three different AR storybooks. At the pre-test all groups read the coursebook texts in a traditional manner and then answered the comprehension items. At the post-test groups read the texts in accordance with their specific AR design and answered the items. In addition, the students completed the cognitive load survey after reading AR storybooks. The results of the within-group evaluations showed that the use of AR content had a strong effect on their reading comprehension. Therefore, it can be stated that augmented reality storybooks increase the reading comprehension performance of the children aged 11-12. In the comparisons between the groups, it is found that the comprehension scores of Group-2 (24-27 s) and Group-3 (37-39 s) were significantly higher than Group-1 (11-13 s). In addition, the cognitive loads of Group-2 and Group-3 are significantly lower than Group-1. On the other hand, it is found that the group with the lowest mental load was Group-2. These results suggest that the duration of AR content while reading a storybook has an impact on the students' comprehension and cognitive load.