How many "office hours" are common (or standard) for full-time lecturers? In situations where one teaches purely undergraduate students (20 hours per week, 16 weeks per semester, 2 semesters per year) how many hours per week, and how many weeks per year, are people normally in their offices? Does this number change after the main teaching semesters complete?

I have just finished my fall semester and I find my normal office hours cannot handle the demand from the students. I'm a bit torn. On one hand, I want to support my students as much as they care to be supported but I must balance my own needs. I have plenty of other things to do besides teach (I have to prepare for the spring semester and that will take me significant time, I have marking to do, etc.). More office hours means less time (thus less quality) for prep (and other various tasks).

When I consider what my peers do, I see they often do less than the school requires of them (I am not interested in following their example). With > 300 students, it seems the only way to really satisfy the demand is to be there 20 hours per week...which leave me little (no) time for my other tasks.

My goal here is to try to identify what is fair to all involved.

Edit: I've posted a follow-up question to this one.
My undergrad university held regular "helpdesks" in a classroom - where one or more teaching assistants and  sometimes the lecturer were present. They essentially functioned like an extra (and optional) tutorial class, and the more proactive students would come in and work on their assignments, discussing among themselves and asking questions.

This allows you to spread time a bit more fairly among students than at an office hour, where one student might take up a lot of time and leave others waiting at the door. Answer one question, and then move on to someone else.

Perhaps you could try a similar strategy?