The shelter is needed for only about 2 months a year. I usually bring it out in the end of March or the beginning of April, one or two weeks before my apple trees blossoms. By late May or early June when the blossoming period is over and tubes are filled, I bring it inside my garage, where the eggs hatch and develop into pupae. By next spring, I will bring it out again, so the cycle begins.
As shown in the picture, the tubes in the attic were filled from last year. The tubes in the blocks are new. When the weather warms up, the bees in the filled tubes will break the seal. Each female will claim a tube and begin laying eggs. It's important to use new tubes every year to avoid parasite buildup.
Mason bee is unique in the sense that each female is able to lay eggs. Unlike other bees, there's no "queen".