How's this - I'd expect to see advantage or disadvantage come up at least once in every 10 rolls. In a low level combat that might be every turn (5 PCs vs. 5 foes with 1 attack each) while many other activities are less roll-dense.
To me, every round in combat is "common". Others may say 10% of the time is "rare".
As a side note, the classes that can reliably generate advantage all pay a cost or have other limitation. Barbarian's "get advantage in your attacks, grant advantage in ALL ATTACKS AGAINST YOU" is an example. Or Assassins have a limited window that they may not take advantage of every combat. Casters have spells that grant it but those have saves and use up a limited resource. And take an action for no/little other effect. Having a PC who otherwise has no costs for advantage and being able to reliably grant them advantage on their multiple attacks with a d12 two handed weapon (not even including Great Weapon -5/+10 to keep it without optional rules besides a new class) is going to make a very effective killing machine.
You mean like an Archfey Warlock casting Greater Invisibility on them every encounter (doable from level 7)?
