As often as they desire. There is no "should" other than they SHOULD have it as often as they want to. Whatever works for them. For some couples it's daily (or nearly so)...some a few times a week, some a few times a month, some occasionally, some rarely, some so rarely they forgot what it's like.
You can't set a standard or guideline and say people SHOULD have sex at "X" frequency. It varies. Hopefully they are having it as often, with each other, as they'd both like to. ☺♥☻ (and I hope to say that about my lover and I someday)