How Long is too long

Even with a good WD hitch properly setup I would not do it. Loaded with family coupled with P metric tires and a really soft suspension combined is plane asking for problems when, not if you find yourself in a sticky situation. Just loaded with family 1500 Suburbans typically are at their weight limits without adding hitch weight.

I'm not sure what you mean by "but could struggle to stay within payload with a 31' trailer on a Suburban.

I'd be more concerned with weights than length. We tow a 35' unit with our Suburban 2500 and wouldn't blink at going longer if we wanted to. We have no issues whatsoever. That said, we have the 2500 'burb with the 8.1L engine and the 4.1 gears - so we are well within all of our ratings (8k trailer weight out of a 12k tow rating, 1,110 lb tongue with 2,200 pounds of payload, etc...). My concern, on your end, is that you MAY be able to stay within your tow ratings, but could struggle to stay within payload with a 31' trailer on a Suburban.

I'd focus on those numbers, instead of length, and see how they play out - that will tell you a lot.

It has the 5.3L and 3.73R from what I gather it's rated at 7400

Have you measured and then checked wheelbase charts?

It depends on how the Burb is set up. I towed a similar sized trailer with a Dodge conv van. But it had the 5.9L and 3.90 gears. If the Burb is properly equipped it will do well. We need more info such as motor and gears as well as what it’s rated to tow.