I'm an American and have a Canadian girlfriend. I have visited her twice in Canada both for a time period of one month. She has visited me three times in the United States with varying duration (couple weeks upwards to one month).
From my experience, they will ask you a lot of questions at customs especially if it is your first time visiting. I don't see it as a major problem since you are American and the policies aren't as harsh, but be prepared to answer some pretty specific questions.
I don't remember too many specifics, but off the top of my head they wanted to know a lot of personal details about yourself (occupation, current living arrangements, family) as well as information about your significant other (how you met, occupation, living arrangements, family members, etc). When they asked my duration the first time I was visiting (one month), the customs officer made a small little comment that it was a pretty long time to visit, but you shouldn't have to worry too much about that. They will ask you how much money you are bringing, I believe I had around $700 USD that I brought. They did ask me if I had a return flight booked already and I told them that I did, I don't remember if they asked for proof or not (don't think they did).
But other than that, you shouldn't really have any problem with the visit as long as you have no intentions on staying for more than the allowed time. Some of the questions may seem intimidating at first, but you'll manage. The second time I went through customs it was pretty straight forward since I already visited once prior and they asked some basic questions and it wasn't too much of a hassle.
EDIT: I forgot to mention that I did say I was visiting my girlfriend and that she lived with her parents. They asked how we met and what her job was and I believe something about her address, but it was all basic information that you should hopefully already know before visiting.