Weeks reset, all buildings are working full steam. 4 more need to enter the following week (total 5) and each week hereafter so it remains open. Double upkeep is no longer required, 1850 lumber a week is all that's needed to pay for upkeep, however if we want to upgrade our buildings, more will be required.
edit: guess we have to upgrade the citadel, nothing else can be upgraded until it reaches tier 2. Need 12,000 lumber.