1Cor 7:5 (NKJ) Do not deprive one another except with consent for a time...So, it is a sin to refuse sex to your spouse. More to the point are the verses just before:
1Cor 7:5 (Wey) Do not refuse one another, unless perhaps it is just for a time and by mutual consent...
1Cor 7:3-4 (NRS) The husband should give to his wife her conjugal rights, and likewise the wife to her husband. For the wife does not have authority over her own body, but the husband does; likewise the husband does not have authority over his own body, but the wife does.As to frequency, I suppose people differ. But it is not for an outsider to say in any case. If one partner wants sex, then God's command for marriage is clear enough: "no refusal" is permitted unless mutually agreed to for a time of prayer. That is the command of scripture. Your body belongs to her, and vice versa.
First, search your own heart and see if you have not "destroyed your own household" (Pv 14:1) in terms of normal sexual response and desire. We deal with this in another posting, but for our purposes here you must honestly ask yourself the question if you have contributed to her frigidity. Have you brought pornography into your marriage bed, for example, or thoughts of another woman? Have you been a lover or just a user of your wife's body? In other words, if you have been sinning and are just reaping the harvest of your ways, then there is no time like the present to repent, ask for forgiveness, and seek God for healing. But if your conscience is clear, and she is sinning by "holding out", this is also a grievous sin that directly disobeys the word of God, and she must be confronted about it.
Practically, this needs to be worked out some other way than by the letter of the law; but the law has its "ministry" (of condemnation). The law won't change her, in other words, but will serve to bring proper conviction into her life. Confront her with the word, in private, as commanded by Jesus Himself. If this does not work, bring two or three to confront her of her sin. Hopefully, she will see her sin and repent. Perhaps the shame of having her secret sin exposed will goad her to take seriously her covenantal responsibilities. God has told you what to do if you find your brother (or sister, in this case) in sin (Mt 18:15-17). Go to her, just the two of you, and confront her. If she does not repent, then go with two or three. This is the command of your Lord and Saviour Jesus Christ.
More than a few women have a problem in this area. Not to be overly dramatic, but these women are prostituting themselves within marriage. For one reason or another, such women like to have their husbands sex starved so they can blackmail them, control them, humiliate them, dole out the favors on their terms, get what they want, etc. They are prostitutes, in other words, who happened to be married. Husbands can do the same thing, and have, as a form of control; but it is more often women who fall into this pathology of sin. Of course, there is always the excuses and contingencies, as with any sin. But God looks to the heart of the matter, to what is really going on. Whenever sex is bartered, it is prostitution; and no money need change hands. Married people belong to each other. A wife in disobedience has become a married prostitute with a single customer. God is not mocked. Such is a great offense and thus the warnings of the scriptures above.
There is not much that scripture says to married couples, and this is something very clear. It is a sin to deny the partner in marriage. This must be confronted and repented of.
Eph 5:3-17 (NIV) But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God's holy people... Let no one deceive you with empty words, for because of such things God's wrath comes on those who are disobedient... Find out what pleases the Lord. Have nothing to do with the fruitless deeds of darkness, but rather expose them. For it is shameful even to mention what the disobedient do in secret. But everything exposed by the light becomes visible, for it is light that makes everything visible... Therefore do not be foolish, but understand what the Lord's will is.