Generally, Zen monasteries divide the year into training periods, and non-training. Training periods are called Seichu. At Mt. Baldy, they have summer and Winter Seichu, lasting for about three months each.
During Seichu, we'd get up at 3:30 A.M. and go to bed by 9 P.M., unless it snowed, then we'd get up at 3 A.M. to shovel snow. We'd have three sitting periods a day, a talk by Roshi, chanting, three meals and work, usually physical, such as cleaning the Zendo, shoveling snow paths, working in the sewing room.
Then twice a month, everything would change, with a 7 or 8 day "intensives" called Sesshin, with at least four meditation periods and 4-5 visits to Roshi telling him of your koan work.
These Sesshin were VERY intensive, with lots of sitting, being hit by sticks on the shoulder if you even got a little sleepy or restless, and in the winter, deep snow on the ground, and no heat except for the first and last 15 minutes of the day. When the temperature hovers around zero, it becomes a big deal.
Besides the physical stress, the other big stressor is that the attitude of the monks changes, from a benevolent ignoring of you, to a raging and angry attitude guaranteed to push your "How dare you say/do that to me" button several times a day. I used to think of burning down the Center during periods in between very deep sitting sessions.
Well, my friend overheard someone who asked his teacher about resolving her unhappiness in her marriage, to which the Zen master replied,
"Marriage is not about happiness, it is Sesshin!"