I assume that what your ammeter is reading is charge to the battery, not alternator output. The alternator actually runs flat-out at all times due to the nature of the regulator. What you are seeing is the 'excess' current available to recharge the battery after using the starter & at tick-over speeds. As I suspected, it does not take long for this charging current to reach zero. If the alternator was undersized, this charge time would be significant.
The ammeter is connected in series with the main fuse so that it measures the current into and out of the battery.
There has been much discussion on this and other forums over the years about the wasted alternator output. It was done that way because it was cheaper and the Japanese didn't invent regulating the voltage that way, but at least they made them last longer a lot than the Lucas (the P.O.D.) ever could.
I am waiting for some whiz kid to come along and design a regulator that does not dump the excess to ground. Maybe something based on a three terminal regulator?
As you also point out, you have extra loads & I guess sometimes you have the machine ticking-over for lengthly periods as you have time to observe the battery voltage falling.
When I first had my GL500 with the original stator & battery I could run at typical speeds in the 80-100 Km/h range all day with loads approximating the original setup with the voltmeter reading a bit over 14V, but when I switched on the handlebar heaters (3A) plugged in the electrical anti-fog helmet shield (1.5A) and turned on the 55W sidecar headlight the voltage would drop below 13V. If I ran like that all the time (the coldest, darkest part of the winter) within a week or so the battery would start to have trouble starting the engine. In those days I typically charged the battery every week in Jan & Feb and at least once per month the rest of the winter.
When I changed to the lawn tractor battery the frequency of charging became lower and when I put in a GL1000 stator it became even less, but I still had to keep an eye on things.
I never ran the 650 with a bike battery in the winter, but with the lawn tractor battery and the original stator it needed charging about as often as the 500 with its original stator and a similar battery and changing to a GL1100 stator yielded similar improvement. Last year I finished converting the 650 to HID headlights and LEDs for all other lights so I anticipate needing to charge it even less often (I was unable to find out this year because I was recovering from tendon transfer surgery in my thumb and unable to drive (or go to work) from mid January to mid April).
My own use is very light compared to yours as we don't have any significant trafic on the 'B' roads (that's one step up from goat tracks) we normally use on ride-outs. We don't often stop for more than a few seconds at junctions & the motor is normally running at 3000+ rpm. Most of us here are not brave enough to go out when it's dark. I would suggest that you & I are at the extreme ends of the current demand spectrum.
Not as much as you might think. My bikes are primarily used for commuting, the GW for summer and the 650 for winter, both pulling sidecars. Unless it has rained I usually take the (dirt & gravel) concession roads and in my 20-25 minute drive to work I sometimes see as few as 5 other vehicles on the road.