How Long is too long

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.