A memory system or flash card may be exposed to elapsed time or increased temperature conditions which may degrade the memory. For example, extended time periods or high temperature conditions may hinder data retention in a memory device. An estimate of elapsed time and temperature conditions may be useful for memory management. An algorithm that periodically identifies one or more sentinel blocks in the memory device and measures the data retention shift in those sentinel blocks can calculate a scalar value that approximates the combined effect of elapsed time and/or temperature conditions.