I have observed an initial drop of 1-2 bars within the first 100-200 miles. After that drop it has held quite steady, even after an additional 500-600 miles.
I've had a different experience than that based on the nature of the trip. I've driven between California and Nebraska a couple of times, which involves some extended uphill climbs and a few serious pushes over steep summits along I-70. In HV mode, the battery is apparently used to supplement the ICE fairly extensively, because 10s of miles of EV range disappear during the mountain drives.