Owners manual on my 2014 Ultra Limited says 2nd-15mph, 3rd-25, 4th-35, 5th-45, 6th-55 mph. Seems a bit tame but I suspect it has something to do with fuel economy. + 10 mph sounds better and no strain.
As others have already said, stay above 2000 rpm at all times. As a general rule, I make sure I stay above 2000 rpm on flat road or going down hill, and above 2500 if going up any incline at all. Anything below 2250 starts to feel a little too low of an rpm for my motor.
I'm in the Houston area so almost all my riding is on the flat land. I generally shift to the next gear when I get the tach up around 3K rpm. However, if I need to get on it and accelerate quickly, I may drop back down a gear so as not to lug the engine under load.