Depends on your drop. I used to clean my 500 gal fish tank with something similar and used 8'. Some of the youtube videos are using only a 50' garden hose which I do not think would produce enough to justify the set up. Most are between 150' and 250'. I think I even saw one at 500' feet. You actually want to have the shortest length of hose you can get away with to give you the drop in elevation you need to move the material you are working. Every location will be different.
The biggest variable is how much of a drop in elevation you can get between where you are sucking your material and where you can set up your sluice. It works at any drop, but the bigger the drop the better the suction. Personally I would want to run at least 10' drop and a minimum 1" hose. Areas I want to try I should be able to get a 20' drop over 50' run which would give me plenty of working space. Since this is just a big siphon you cannot raise the suction end out of the water while moving around or you would break the suction and need to set it all up all over again.
I wont be able to try this out until Oct or later when I can head to northern NM. For me it is a possible way to dredge without the major expenses of buying or making a motorized one. Dredging is still legal in New Mexico under certain regs, but where I am there is either gold or water not both. For those in Cali it just may give a option that is still legal. I would certainly print out the Cali laws and regs however so that mean old uncle sam does not try bullying me around.