I typically get 35 to 38. I have the tall windshield, but it isn't clear that that hurts my mileage because when I had the stock shield installed, it was in the same range.
Really, as others have noted, the most important factors in your mileage are whether or not you are cruising or doing city stop and go, and how you're using your right wrist.
Really though ... this is not that much different from what I've gotten riding a wide range of sport touring style bikes. My FJR 1300s have all gotten 40ish for example. The R3T is very heavy and it has a pretty large frontal area to blast through the wind. High speeds and climbing will use fuel up faster than level cruising. But you take the road you need to take, not the road that's the best for gas mileage.