Is it normal to go through a full tank in one afternoon?

Yeah, that's just the Rocket doing Rocket things. I ran one for two years and plan on 120 to 150 miles between fills, maybe a bit less if I'm having fun with the throttle, which on that bike is basically always. It's a 2.5 liter engine with a small tank, so the math was never going to work out in your favor. My old bike with a third the displacement would do nearly double the range too, same deal. You trade legs for that torque wall, and honestly nobody buys one and then regrets it at the pump.

I'd push back on the 'nobody regrets it at the pump' bit though. Plenty of people regret it the first time they're on a group ride watching everyone else coast past gas stations while they're doing math on whether they'll make the next town. The short legs thing is real, but it's also why half these bikes end up with auxiliary tanks or get sold inside two seasons. That said, a fresh engine does tend to run rich and loose for the first few hundred miles, so some of what he's seeing could genuinely settle down. Hard to separate that from a happy wrist though.

Back in my day we didn't have trip meters, we kept a paper log taped to the tank bag. Mileage out, gallons in, worked it out with a pencil at the pump. You'd be surprised how many guys thought they had a thirsty bike when the gauge was just lying to them. Fill it, zero the odo, ride till you're nervous, then do the division yourself a few tanks running. That tells you more than any forum ever will. And yeah, the first thousand will read a touch rich, rings take a while to bed, but not enought to explain a whole tank gone.

New rider here with a dumb question. Mine's a small bike so I can't compare on the fuel thing, but I did notice on a friends big cruiser that the gauge drops fast at the start of the ride and then kind of slows down. Does the needle on these things just sit low for a while, or is the first half of the tank really gone that quick? I get paranoid and stop way too early beacuse I can't tell if the thing is lying to me or not.

Update: started doing the pencil-and-paper thing like the tank bag guy suggested. Zeroed the trip meter at every fill and it turns out the gauge was lying to me, needle hits low early then kind of hangs there. Actual range is right in line with what the two-year owner said. So no, nothing wrong with the bike, it just drinks like it looks.