Measuring opportunities to learn (OTL) is crucial for evaluating education quality and equity, but obtaining accurate and comprehensive OTL data at a large scale remains challenging. We attempt to address this issue by investigating measurement concerns in data collection and sampling. With the primary goal of estimating group-level OTLs for large populations of classrooms and the secondary goal of estimating classroom-level OTLs, we propose forming a teacher panel and using an online log-type survey to collect content and time data on sampled days throughout the school year. We compared various sampling schemes in a simulation study with real daily log data from 66 fourth-grade math teachers. The findings from this study indicate that sampling 1 day per week or 1 day every other week provided accurate group-level estimates, while sampling 1 day per week yielded satisfactory classroom-level estimates. The proposed approach aids in effectively monitoring large-scale classroom OTL.