We've been married going on 12 years, and while our frequency may have slowed our passions have not. We often tell each other afterwards "why don't we do this more often?"
I couldn't give you a specific number because our situation may not be the same as yours. His schedule is so irregular that he may not be home for weeks at a time, or he'll be here during the week, but not Thursday through Monday - crazy stuff like that. So we make do.
If I'm not in the mood then nobody's in the mood. OK, maybe not that bad, but it's rare that I will feel responsible to satisfy him because he's in the mood and he brings home the big bucks. It don't work that way. And vice versa. If he's not in the mood then I don't push it.