What a beautiful plant, Avedon.
The life span of rosemary more than likely depends on the zone, and yours might grow well forever. I've read that it can live for 30 years or more, but that would have to be in a zone that is more like the Mediterranean where it originated, I think.
I'm not sure about your weather, ours changes drastically in a matter of minutes here and one of the reasons mine does better in a sheltered location is because of that. On a mild sunny day in January it dropped from 70 degrees at 3 in the afternoon to 35 degrees and heavy rain by dark and by midnight was in the teens and frozen solid with more ice falling. I remember because I had washed outside windows that day and didn't finish. It isn't that it needs shade, it's that it needs a bit of shelter at least on one side. My guess is that if it is planted in the open but near something like a wall of your house, or in my case, a column like structure - anything that gives it a little protection - it will survive for years for you.
Even knowing that, I still keep taking cuttings and trying different spots often. If I were you, I'd clip a few cuttings just in case. It's such a pretty plant, too.
Let me know how it does. Cuttings are very easy to root.
