You are unlikely to get a precise answer to that question, as it varies with the design and construction of the charger. In more or less all cases, however, the answer is extremely short and usually physically sitting on the charger.
How you rest it on the charger makes a difference. For the DT-910 I found the phone had to be in landscape orientation and about a cm above the base. I use a small item to rest the phone on to maintain charging. The DT-900 requires use of something to hold the phone in place. Without that it slides off the charge plate because the back is so slick. I use the device cover that shipped with the phone. In all instances the phone is resting on the charger.