I work at a huge firm, but within the firm my department is only like eight people. I also arrive way before most others do, so in the mornings I greet the security guard when I come in and say hi to the co-worker who starts when I do, then say hi to my other co-workers when they come in. That's about it. If I see someone when outside our little office room and they say something to me, I'll say something back.
When I was in Japan, I didn't greet everyone every morning, either. Did basically the same as the above. The exception were the kids at the schools I worked at. I'd say hi to all of them because that was part of the job, in a sense.