The Bible doesn’t tell us how often a married couple should have sex; it does tell us that a couple is to abstain only when it is a mutual decision. First Corinthians 7:5 tells us, "Do not deprive each other except by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control." So, mutual consent is the "rule" for how often a married couple should have sex. The "rule" is that abstaining from sex must be agreed upon, and that even when it is agreed upon, it should only be for a short time.
Sex should not be withheld or demanded. If one spouse does not want to have sex, the other spouse should agree to abstain. If one spouse wants to have sex, the other spouse should agree. It is all a matter of compromise. We must remember that our bodies belong to our spouses, as 1 Corinthians 7:4 tells us, "The wife’s body does not belong to her alone but also to her husband. In the same way, the husband’s body does not belong to him alone but also to his wife." Obviously, the "sexual compromise" in marriage must be reasonable. If one spouse desires sex every day, and the other spouse once a month or less, they will have to lovingly and sacrificially agree to a compromise, a middle ground. Studies show that taking into account all age ranges, a typical married couple has sex 2 times per week.