In summary, the more the better.
Although it has to be said that this is usually not an accurate way to determine the happiness in a relationship because some people have learnt how to cover up their internal issues with external shows of love and public displays of affection.
Regardless there is still a lot to be learnt from observing the way partners speak, touch, and generally relate with each other.
Having said that, this is what the regularity of sex says about your relationships:
1. Twice a day
This is “typical of a very very very happy relationship – relaxation/honeymoon ecstatsy.”
“Typical of a very happy marriage.”
3. Every other day
“Typical of a harmonious marriage, especially in working couples.”
4. Once a week
“Typical of marriage experiencing pressures of life”
5. Twice a month
"Typical of a disharmonious marriage, especially life in-between quarrels/storms."
6. Once a month
"Typical of a marriage in co-existence – uninterested in each other."
7. Once two-three months
"Typical of a marriage in serious disharmony – unfaithfulness/adultery."
8. Once a year
"Typical of total breakdown of marriage and virtual divorce."