Animals that can appropriately adjust to varying environmental and nutritional conditions possess a survival advantage. Maintaining homeostasis and homeorhesis in response to changing nutritional conditions requires flexibility in nutrient partitioning and use efficiencies. This is especially the case for protein metabolism because there is no dedicated pool of amino acids held in reserve for times of changing dietary protein availability. Research on the adaptation in protein metabolism to changing dietary conditions could have potential implications on experimental design and on basic and applied ruminant nutrition. However, experiments directly evaluating the metabolic characteristics and appropriate timing of dietary adaptation are very limited. This is especially the case when compared with the number of experiments that have evaluated a response to a nutritional change after the period of adaptation is presumed to be complete, although rarely verified. Factors affecting the adaptive responses in protein metabolism in the ruminant are multiple and likely interacting, although the time required for adaptation appears to depend most on factors such as the duration and level of the previous nutritional condition, the difference from the current nutritional condition, the timing of these dietary changes with respect to the physiological state of the animal, the priority of the metabolic demand, and the productive state of the animal (i.e., the respective combinations of maintenance, growth, and/or lactation). As a consequence of these varied factors, from an experimental perspective, the time required for adaptation may depend on the response variables of interest, productive state of the animal, and the treatments under investigation. Existing literature will be reviewed to highlight time-related adaptive responses to diet changes on protein digestion, post-absorptive metabolism, and production and to emphasize areas where this understanding is incomplete.