Happily married folk and the amount of sex they have will vary but on average I would say once a week is pretty accurate.
Unhappy married folk or those just co-existing and staying married 'for the kids', which is a lot, have sex very little. For some it is literally just on birthdays or anniversaries when they feel they need to make effort. I was shocked to hear from 3 of my married friends at our recent guys 50th reunion long weekend that 3 of them, most of consider to have good marriages have no sex whatsoever and have not for years. I was shocked at how easy they talked about it. But then I have a neighbours here in Edmonton and the wife loves telling others, as soon as she has had a few drinks that her and husband have not had sex for over 6 years.