After Decarbonzing many 100s vehicles, from my experience it should be done in the 1st 19,000-20,000kms . after as a maintenance procedure it can be done every 10,000km.
Anything before 19,000kms is difficult to experience a difference in the vehicle performance.
But an emissions test before and after can be done to verify the decarbonization procedure even if its done before 19,000kms.
In INDIA unfortunately there aren't any Service Centre that has proper equipment to do this service. None of the Mechanics are even qualified or properly trained.
First of all we need to understand why is there carbon formation in the engine.
1) regardless of where you purchase your fuel, it is of low quality (regardless of the country)
2) Maximum OCTANE / RON rating for FUEL at Fuel Pumps is 100. In India I think its 95 / 97 at Shell.
3) So what does this mean? (OCTANE / RON) The higher the value the more effective the combustion of the fuel meaning more power to the engine and lesser wastage of fuel.
4) What wastage? Because no internal combustion engine in the world is able to burn fuel completely and there is wastage. Not even race cars or Rolls Royce etc can burn fuel completely. So this un-burnt fuel become carbon formation inside the engine compartment.
5) For Example: 1 liter of PETROL / DIESEL is sent to engine for combustion, only 700 -800ml gets burnt. The rest of the fuel is un-burnt and starts to form carbon deposits on piston top, spark plugs, glow plugs, piston groves, exhaust manifold and even catalytic converter.
6) So spending money on Decarbonization liquids by pouring into your engine oil and using fuel injection cleaner solution in the fuel tank won't solve the issues caused by carbon build up.
7) How to properly Decarbonize?
A) Take engine apart and physically remove all carbon deposits by chemical solutions or B) Use High Octane Gas without taking apart the engine.
8) Fuel Injector cleaning has to be done by Ultrasonic cleaning by removing the fuel injectors and soaking them in an ultrasonic bath and doing a test of the spray patterns before and after cleaning them. Fortunately BOSCH service centres do this service on request.
9) Why fuel injectors get dirty? Dirty contaminated Petrol / Diesel. Just like we have water filters / purifiers at home, we also need a proper filter when re-fueling our vehicle. Our Fuels are contaminated with sand, dust particles and of course water.
Diesel is especially hydrophobic (attracts water) so diesel fuel almost all the time has water inside.
10) the only way to ensure fuel is filtered is to used a fuel funnel filter before refueling.
11) why is there black smoke is Diesel vehicles? Unlike Petrol engines, Diesel engines run a rich Fuel and Air mixture. This means more fuel than air. As time passes the un-burnt diesel becomes carbon formation as well as black smoke. So even cleaning the EGR Valve would not help to reduce black smoke.
I have successfully Decarbonized many Diesel vehicles with a reduction of up to 80% of Black smoke from the exhaust.
Have emissions test results to prove as well many positive sms from clients who did Decarbonization with the PHD60 Treatment.
All typed here is based on first hand experiences with cars as well as bikes in India and Singapore.
My Fuel Funnel Filter
The only TUV Tested proved working Decarbonizing Procedure in INDIA http://www.fb.com/phd60