I think it varies from marriage to marriage, and from time to time within each marriage. When DH and I first got married, every other day was the norm. Now, he starts whining if it's less than once a week, which I consider a practical "goal." So that's what I try for (my sex drive is nonexistent while pregnant).
Luckily my DH is very sensitive and understanding (and it sounds like yours is too). But I do still try to make sex a priority when I can - it is an important part of marriage (moreso to men I think, but I realize that may be a generalization).
Stacy
Wife to K
Mommy to A (5) and twins E & S (1.5)
The biggest mistake I made is the one that most of us make...I did not live in the moment enough. I wish I had not been in such a hurry to get on to the next thing: dinner, bath, book bed. I wish I had treasured the doing a little more and the getting it done a little less. - Anna Quindlen