[Old man mounting soap box] Before "gravel" bikes were a thing, we all rode cyclocross bikes with 32 or 35mm tires and nobody complained. [Dismounts soap box]
I generally run 40mm tires, some of which will actually measure at 43 or 44mm. I still have a set of 35mm Kenda Alluvium Pros that I really like for mixed surface rides, but that can still handle the nasty stuff. Frankly, if I can't ride something on a 40mm tire, then I want a mountain bike - not so much for the tire width, but more for the flat bar and the frame geometry. I'm in Connecticut, and my riding can range from smooth, maintained dirt roads, to single track trails, to washed out stream beds.
But right now, I would say 40 is my sweet spot.