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I've used a SON 28 dynamo, a Busch and Muller E-Werk converter, and a small buffer battery for about 6 years. Travelling at 17 km/h, you get about 2.5W, 500mA@5V for charging USB devices. I carry 2 x 13,000mAh Anker battery banks. So to fully recharge one of those batteries, I have to travel at 17 km/h for at least 26 hours, and that doesn't account for inefficiencies in the charging process, so more like 30 or more hours.

My average speed on long multiday bikepacking trips is roughly 11 km/h (I'm not fast). That's fully loaded (35 kg/ 77 lb bike, gear, water, food) with lots of climbing, so sometimes that speed is more like 5 km/h going uphill and sometimes 30 km/h going down. So a rough estimate is that charging one of those batteries will take about 40 hours of typical riding. That's 3-4 long days. With my setup, charging an iPhone 13 Pro, Hammerhead Karoo 2, Bluetooth earbuds and batteries for my point-and-shoot camera, I can stay fully charged for about 2 days per battery bank. So with 2 battery banks, I get 4 days and possibly as much as an additional 2 days from charging from the dynamo. Or another way to look at it is that I extend my time by about 50% without any recharge from an outlet.

You can extend the usefulness of the dynamo if you are more careful than I am with a phone and if you have a more energy-efficient bike computer than the Karoo.

If I were making the decision on whether to use a hub dynamo, given what I know now, I'm not sure I would. In my bikepacking, I've rarely gone more than about 3 days without finding some way to plug into AC power and recharge. The one thing that might still tip it over to a "dynamo yes" is that you can always have power, however meagre it is. So if you need to ride at night or need to charge your phone to get some signal, you can generally make that happen, at least if you are not going uphill!

This is definitely a "your mileage may vary" situation, but at least I can provide a long-term example.