Really, the best overall answer would be weekly. For a number of weeks in a row, probably a minimum of six weeks. Why? If you are going to take the time and expense to go to couples therapy, it would be best to optimize your experience. Going at least once a week for at least six weeks means that you will be really digging into the work, your work, the two of you, and what is going on between the two of you. Sometimes, when couples have less problems that are less severe, that would be enough, six weeks might be all that you would need for significant change. If there are deeper problems (financial, sexual, extramarital, familial, etc.) issues, you should be into the thick of it by six weeks of therapy, and the couple’s issues should be opened up by then and being addressed. You will be in the middle of couples counseling, seeing some change, seeing your part, seeing what more you need to be doing.
What if you can’t go every week? It is not the optimum, but can work, and work well for couples. Sometimes it helps to go to couples counseling for two sessions in a row (back to back, about 100 minute sessions) if you can’t make every week. Double sessions will add intensity when the schedule is not doing so.
Some couples therapy is better than none. If you can only go irregularly, that will still work. Meeting less often means that the couple has to do more work on their own in the interim periods, to think about what they each need to change in themselves, and to hang onto that rather than to use the time to slip back in to old ineffective and/or destructive patterns, to slip into the pattern of blaming the partner.
So: think about going, about doing the work, about changing the couple and changing yourself on a weekly basis if you want to enter into couples therapy. There is a lot of research over many years that shows that it helps couples change. Which is what you are after, right?