Theoretically a gutter vac can go as high as a) the operator can control it and b) the poles can stand the strain.
Assuming the average 5ft aluminium pole section weighs about 1 kg then a 50 ft reach would be 10kg which is easily liftable by most fit people.
But this applies when the pole is completely vertical. As soon as it starts to lean the operator is faced with a new force which is a turning moment. He compensates for this by pulling with his upper hand and pushing with his lower hand. The more the pole leans, the greater the force he must apply. Holding a 50ft (15m) pole horizontally would put huge strain on any operator, not to mention the obvious deflection (bend) in the pole itself.
Personally I would not use alu poles above 35ft, but I perceive that to be my own limitation. Some people do go up to 50 and even 60 ft. If you are intent on doing this then I would suggest practising with the pole in a place where you can't do any damage to yourself, others or property, gradually adding sections until you lose your 'comfort zone'. This will help to tell you how high you can safely go. I would also suggest that anyone poling above 35ft should have an assistant who can move the vac, help the operator and generally be there so the operator can keep full concentration on the task of cleaning. The operator will also get tired very quickly so either frequent rests or relays between two operators.
Another solution is a lighter pole. I've seen carbon fibre gutter poles advertised which are maybe 1/3 of the weigh of aluminium. Very expensive but you can charge a premium for high work so it might be worth it. Up to you to decide.
Other considerations are loss of suction with a 50ft pole so you'll need a good vac, no wind at all so you'll need a nice day and how tricky it is to guide the nozzle into the gutter and then move it along at such a great height.
Final word if you're still reading. The one thing every gutter cleaner fears is a dynamic pole. A dynamic pole is one that the operator has lost control of and is now hurtling towards the ground, just seconds from its own destruction and the destruction of the Aston Martin beneath it or worse still the Aston Martin and the owner of the Aston Martin. I've never seen it happen and I pray that I never do. But gutter cleaning carries a huge responsibilty so please make sure you are in total control at all times. There's no shame in looking at a job and then saying, "Sorry, I can't do it".
