A lot of the cleaning that you would hire professionals to take off of your hands and into their more capable ones will require a fair bit of time to see through. You would do well not to rush your hired experts because of the fact that doing so would force them to cut corners to meet your unreasonable demands. It is far better to just let them do what you have asked them to, since they know better about how long the task should take once all has been said and is now out of the way.
That said, it can be a nightmare to work around people that are cleaning your air duct on your behalf, which means that understanding how long this process takes on average can be a very fruitful piece of information for you to acquire. If we were to speak of this matter in a general way, we would say that it usually takes around three hours in order to make sure that your ducts are clean as can be. Trying to force your service provider to do it in less time will result in them doing a worse job than might have been the case otherwise.
There are a lot of cases in which air duct washing required as little as two hours, but on the off chance that you have let your ducts stay dirty for far too long you would likely need to wait the full three. If you were to get this done as regularly as you can, your professionals would know just what to do and your wait time would drop to two hours or so.