Kefir grains can be left in milk for 36 hours in the fridge for a week. Milk kefir grains remain happy as long as it takes them to consume all lactose. That can be completed faster if the ambient temperature is higher and the ratio of grains to milk higher.
Once all lactose in milk digested by the grains, the fermentation process completes. Further presence of kefir grains in the kefir or generally leaving kefir in room temperature will cause separation and grains starvation.