The answer to the question, by anyone with any sense, is "how often they want to have sex." I mean, seriously, who cares if your frequency matches someone else's frequency? Sex, marital sex at least, is not a league sport.
The studies are popular with one of the two partners in most marriages. In many marriages there are two very different sexual attitudes. One partner really enjoys sex and wants to have it more often. The other one doesn't like sex as much and would like an excuse to have it less often. The studies become more ammunition for whoever's position is supported by the most recent finding. If you are the "non-sex" partner, then you are glad if the study shows you are "beating the average" because you can argue for a reduction. If you are the "more sex" partner, then you are glad if the study shows you are "below the average" because you can argue for an increase.
That's how low we have sunk in our marriages. We resolve sexual issues by referencing the second-hand reports of how often people we do not know have sex. Then, for some reason, we think that our partner ought to "conform" in some odd way to the reports of the things these strangers do.
The Bible has a much simpler approach. Each partner is to have sex when the other partner wants to have sex. 1 Cor. 7:3-5. Pretty straight forward, isn't it? That is the rule. Sex is never about percentages or strangers or studies, it is about the man or women to whom you are married. You are the sole and total source of their sexual pleasure and are to be available to them.
This, after all, is of the essence of the command of God in Proverbs 5:
Let your fountain be blessed,
and rejoice in the wife of your youth,
a lovely deer, a graceful doe.
Let her breasts fill you at all times with delight;
be intoxicated always in her love.
We live in a time of greatly increased access to pornography, in which we see increased frequency of divorces, in which we see common commission of adultery, and in which we actually think that some study of strangers is relevant to sex with our spouses.
Men and women, husbands and wives, need to love one another. They do not need to read more studies.