Define 'normal'
What's normal for some will be different for others.
I find that there's no average times per month/week etc, it's more a case of when the moment happens. Having said that we both work long hours and are usually exhausted and want our bed solely for sleeping
I also think that age comes into it, the younger you are the more often you're 'at it'... at least that's my personal experience. Having said that, when you do, it more than makes up for the lack of it