That just depends on the couple. My wife and I have sex 3-5 times a week.
"Most couples, they have sex at least - well starting the first couple of years, it’s three times a week or more. And after two years, most American couples have sex two to three times a week. That’s the average. Now of course the range is from zero to several times a day, so you can never say what’s normal for a particular couple, and by normal I mean average. So that there are couples who have sex three or four times a day through their whole marriage."
I took that quote from the articule in the link provided below. There is a ton more info, if your interested.
Just for the record, I'm 30 and I've been with my wife for 10 years. I get very crabby if I have to go more than every other day without sex.