Lori, I think that the snowman has summed it up pretty well by saying that poly is a state of mind. Trying to analyze the differences between swinging and poly by looking at quantifiable statistics like the duration of a relationship isn't going to get you any closer to understanding anything.
Look at it this way: normal people in theoretically monogamous relationships generally sleep around. Swingers do too. Looking at the behavior, there isn't much difference other than that swingers usually do it in groups. Socially though, the big difference is that swingers don't hide the sex from their partners, they share it with each other. Swinging is a state of mind, swingers walk around enjoying the idea that they are sophisticated enough to be able to share in their outside sexual experiences with their partners.
Poly is the exact same thing, but with regard to emotional bonding with other people rather than sex. People in normal monogamous relationships fall in love with other people all the time, the difference with poly people is that they don't hide it from their partners. They share the experience with each other. The whole idea for polys is that it's a wonderful and exciting thing to be able to share the joy of the energy that you feel when you fall in love. Having an intimate sexual partner that loves and supports you while you're learning about somebody new is a pretty powerful thing. It's all extremely romantic and I still say that it's far more powerful than anything that swinging has to offer.
Some clarification on my personal experiences though:
Well, a lot of poly relationships are a lot like going steady. You're on the eternal quest for the perfect relationship that 'sticks' and you try to get there by going through different relationships and giving each one a chance. Some poly relationships last much longer and look more like a real marriage, some don't. My longest was six years and involved the purchase of a house. I've only been with my wife for three years so far, so I do think that it's unjustified to label poly situations as inherently less committed. Poly relationships are more fluid, is all. If it makes sense for one person to leave the group and go off on their own, then that will normally just happen peacefully in a poly situation rather than the forever-or-bust situation that you see in monogamous relationships.
This all overlaps. I personally was involved in poly relationships for about a decade, in a sort of continuous peaceful flow from one configuration to the next. For about six years a common element in my relationships was a particular long-term girlfriend. We went through all kinds of situations together that you could liken to 'going steady' with various people, some lasted longer than others. One of those configurations was with another bi girl, that's the one that lasted for three and a half years. Our threesome was very stable and ultimately only ended because of health problems, one of the girls was hospitalized. A while after that the long-term girlfriend drifted out of my life when our social circle split in two because some of us moved out to Silicon Valley. I tried moving out there but it didn't stick for me, she was a computer scientist also and she happened to be falling in love with my best friend, another computer scientist. She left me our house (it was during the tech boom when houses were cheap to us) and went with him, they're traditionally married now and have a son together. That was probably more detail than you strictly needed, but you seemed to be a little confused by my abridged description.
The long-term girlfriend and I were together for six years, through college and through the first few years after college. We started a company together that is still doing good business and supporting my wife and I today, we bought a house together and shared vehicles. When it made sense for us to go our separate ways we divided our shared assets responsibly with no messy divorce battle. We were together 24/7 through some of the most exciting years of our life, we supported each other through college and through striking out on our own in the real world, we treated each others' families as family, we shared sexual conquests and the energy of new relationships with each other, and we explored each of our bisexuality together. It was arguably a far more committed and successful relationship than most marriages.
I'll flat-out disagree with that one. As the snowman says, poly is a state of mind. The emphasis for poly people is always love and the relationship, never sex. Sometimes poly relationships don't involve all-around sex at all. The long-term girlfriend that I mention above was in the habit of dating lesbians for a while, one of them in particular for quite a while. That lesbian and I were pals and we had a lot in common since we were both in love with the same woman, but we didn't have a sexual relationship.
Poly people don't run around flirting all the time or making sexual innuendo all the time like swingers do. They don't emphasize sex and sexuality all the time, they emphasize love and friendship and relationships. They are romantic, where swingers are erotic. They are an entirely different beast than swingers and they generally look down on swinging as being a completely different and very inferior thing.
As the snowman points out, that's a drastic oversimplification. Most poly people would find that statement as offensive as we find it when people come in here and tell us that swinging is just a rationalization for cheating on our spouses.