You are in the silo for 24 hours, but you are not awake the entire time. You take turns with the other missileer sleeping. During the entire alert you are actually working an average of 2-3 hours depending on if there is an exercise going on. The rest of the time people study (kind of like studying in the vault), work on Master's classes, watch Direct TV, play games(I can neither confirm nor deny that there are people who bring PS2 or an XBox on alert). There is a "support" crew topside which consists of a chef (if you want to call them that! lol some are ok, however), a FM (Facilities Manager- takes care of most of the maintaince topside and much of the equipment downstairs, and 4-7 cops.
To answer your question as to why they do 24 hour shifts, to be honest, that is just the way it has always been done. I think it is partly because of the long travel times to get to the sites. Their just isn't enough manning to have shorter shifts. However, during certain "real world conditions and states of readiness" they have an A1/A2 crew which is basically another crew topside that you change out with every 12 hours because both crewmembers are required to be awake at all times...that is just about all I can say about that.
According to regs, you can only have 8 scheduled events a month meaning 8 alerts, 7 alerts and a standby, and so on. Most line crews have the full 8 events, but when you go to a shop (OGV or OSS) you only have 2 alerts a month. Other than the alerts you will have a minimum of 1 4-4 hour sim (you practice ending the world...lol) and 2 half days for other training. So, when it is all said and done I was working at most 12 or 13 days a month. The rest of the time I could do what I wanted.
I am not going to lie, it does get pretty boring at times, but as with any job, it is what you make of it. Most work on Master's degrees. That is the 1 cool thing about the job...your free time to get an education. It is very rare that someone would PCS without a Masters complete. The follow on jobs after your life as a missileer is pretty good. Some go to Vandenberg (schoolhouse or Space lift), Colorado Springs to "drive" satellites, Florida for space lift, and the few that go to ???? for an NRO job. From what I have heard, the guys who go to Colorado Springs to do the satellite thing have a good deal afterwards since the civilian contractors love to hire seperating officers to do the same job and pay them 3 times as much as the Air Force was.
If I didn't like flying so much, Space Command would have been a very good life for my family and I...decent bases (other than the missile bases) and you are non-deployable for the most part.
If you have any other questions, please let me know and I will try to answer them for you.