If an Airbus A380 lands with all brakes disbled, including reverse thrust, wheel brakes and any other measures it has to stop or slow down. Then how long would it take it to reach a complete stop.
This is a very hypothetical question. If the aircraft has no means to slow down it has to lose all kinetic energy through rolling friction and air resistance. Both of these are very small (and get smaller as the aircraft gets slower) and will not have too much effect on a heavy aircraft like the A380 with an enormous amount of kinetic energy. Therefore the aircraft will continue to roll way beyond any runway or proper landing surface.
For high speeds, the air resistance will have the bigger impact on slowing down the aircraft (air resistance is about proportional to the square of the true air speed). Once the aircraft gets slow, the air resistance gets very little and rolling friction (highly dependant on ground surface) will be the bigger force to slow down the aircraft.
In practice, it would keep on rolling (as TomMcW summarises it) "until it hits something". It will run into something before it stops of its own accord.
How far it would roll if there were nothing in the way depends on a lot of things, some of which are discussed below.
Real-life or idealised conditions?
Are you asking about an idealised perfectly smooth and level runway, in conditions of still air? Or are you asking about real-life conditions?
I'm assuming that your disabled measures include the flaps, so now the plane is coming in for an over-speed landing.
Real-life runways are not long enough; under most conditions your plane would run off the end, onto rougher terrain.
Also, real-life runways are also often inclined. Once something like an A340 is moving slowly, its enormous mass needs only the slightest encouragement to keep rolling indefinitely to overcome friction.
Perhaps the closest we can get to an idealised runway is a desert salt flat, but the weight of a landing airliner would probably be high enough to damage it substantially and have a very rough experience.
Then we have to decide: how much fuel, how many passengers (i.e. how much mass has to slow down)? What's the air density? Will the tyre fuses blow because of overheating due to the prolonged high-speed roll?
Too many variables
In short, too many variables, and adjusting just some of those within real-life limits could change the stopping distance by an order of magnitude.
Calculating with an idealised model
A different way of asking the question would be to agree on the weight of the plane at landing (passengers plus fuel).
Then, we decide that it is landing at its minimum possible speed without flaps (180 knots, perhaps).
Now we have its momentum.
It's going to land on a perfectly flat smooth runway, in still air, at STP (standard temperature and pressure: 0 degrees C, 1 atmosphere). The runway has no end.
At that speed, air resistance will help slow it down. An engineer will be able to calculate the aerodynamic drag under those conditions, but there'll be quite a bit at first, and it will drop off rapidly as the plane slows.
We'll assume the tyres miraculously remain inflated until the plane comes to a complete halt. An engineer, again, will be able to calculate the friction of the wheels and tyres.
Our engineer will be able to combine the drag and friction into a calculation that includes the initial mass and landing speed to give us a curve that finally reaches zero.
Under these perfect conditions, the plane will roll for many kilometres. The tail of the curve will be shallow and very long.