How Often

My wife and I are 30 years old, with a very active 3 year old boy, and we both work. I completely understand that we have very little time for ourselves, but lately it's really starting to seem like she has no interest in any sort of physical contact. We might have sex twice in a day, but then nothing for at least six weeks. This has been going on for about 2 years. I'm curious how often other couples with children are having sex, or pleasing each other? I know sex isn't the most important part of a relationship, but I feel like if both parties aren't satisfied it makes it difficult to continue the relationship.