If the queen has been laying well for the last 4-6 weeks or so, the population should be building up now. As it gets warmer, more bees will be leaving the cluster and patrolling the hive to repel intruders.
I use the guards with the 4-5 inch slot, not the one with holes. My bees always use the upper entrance in early spring. I remove the mouse guards when it's warm enough to completly get into the hive and clean the bottom boards; during the last week of March or first week of April. Fortunately, most of the mice are off on their own finding natural dens, and not particularly looking for a warm place to spend the night,.or winter, like a beehive.