It depends who, when where, how old etc.
But let's put it this way as the testosterone depletes and the days get busier there is a lot of other stuff to get done. By the time you have cooked the dinner got 6 kids to bed, done some physics and chemistry homework read your little kids "The Gruffalo" for the umpteenth time you are normally not thinking "let's go to bed and have sex" because you are tired and want to sleep.
Think of it like cooking. When you are first married or engaged and have all the time in the world to cook Lobster Thermidor or travel into the city to see Hamlet or A Midsummer Night's Dream.
When you have 4 children under 8 or 2 under 5, forget about it. You have neither the time nor the inclination.
36 times per year is once a week basically, figuring on pregnancies, post pregnancy, illness, business trips away, late night conference calls, sick kids waking up, one sick spouse, you haven't shaved, you haven't washed for 5 days, she's on the rag, you have a sore back from gardening. lifting, lying under the car.
Less than 12 and you've probably got problems, (in my experience of what constitutes normal) but who can tell because people often lie about this stuff. There's no hard and fast rule, people vary and they establish different normals that they are content with. Water sort of finds its own level. Suffice it to say that as the years pass I've come to the conclusion that quality is better than quantity because if you ain't going to make an effort and light some candles and change the bedding, then frankly you kind of wonder why you bothered. You wake up the next morning and feel the same as if you didn't have sex, so what was the point? There's got to be more intimacy that an Orthodox Jew who cuts a hole in the sheet or there's no pleasure in it for either party.
What I do now, is when I am going to a conference or a business trip in Northern Europe or London I bring my wife along or she flies in sees the town while I am working and stays in the hotel with me. Then we have no kids and no distractions.
I'm taking my wife to a karaoke night for charity on Saturday so that's a definite (since I will also wash and shave before we leave). She loves it when I sing. Russian women are massive romantic fools.