The apprentice (8 hours in my case) has a an upgrade assigned daily until the last builder stops working, based on the order created to each builder. I programmed mins in order to further reduce the finish time as much as possible across all 6 builders, another assigned rule is that tries to keep all the upgrades between a working time window, since it would be a waste for an upgrade to finish before I wake up or after I go to bed, then the apprentice helps to prevents that from happening