by law if I remember the DMV was anything more then 4' past the truck needed to be flagged don't know the max.
Not really sure there is a max for say you just need to get permits and a chase rig.
(a) General Provisions. - Whenever the load on any vehicle shall extend more than four feet beyond the rear of the bed or body thereof, there shall be displayed at the end of such load, in such position as to be clearly visible at all times from the rear of such load, a red or orange flag not less than 18 inches both in length and width, except that from sunset to sunrise there shall be displayed at the end of any such load a red or amber light plainly visible under normal atmospheric conditions at least 200 feet from the rear of such vehicle. At no time shall a load extend more than 14 feetbeyond the rear of the bed or body of the vehicle, with the exception of vehicles transporting forestry products or utility poles.
(b) Commercial Motor Vehicles. - A commercial motor vehicle, or a motor vehicle with a GVWR of 10,001 pounds or more that is engaged in commerce, that is being used to tow a load or that has a load that protrudes from the rear or sides of the vehicle shall comply with the provisions of 49 C.F.R. Part 393. (1937, c. 407, s. 81; 1985, c. 455; 1997-178, s. 1; 2005-361, s. 2; 2009-376, s. 4.)
When in doubt ..flag it.
And for Gods Sake ...if you drive down the road with 3-4 24-40 ft ladders in the back of your truck because you need it for work....BUY A LADDER Rack! This will not apply to ,most here....but I see it every day...I even have a competitor how has a PVC homemade ladder rack...driving a new truck.