A test series with an Huawei Mate 7 and an ST25DV development board was conducted by myself. The ST25DV has an energy harvesting output pin, on which different resistor values where applied. Meanwhile the field was observed with a simple loop antenna and an oscilloscope. The smartphone was attached directly on the ST25 board, making sure the 2 coils are coupled in an optimal way. Here is the recorded I(U) characteristic of the energy harvesting pin:
As you can see the maximum power that can be drawn from the tag is around 750 µW, at a current of 200µA. Trying to draw more energy out of the field resulted in the field breaking down, or rather beeing disabled by the smartphone.
After that another air coil with 20 turns was build and placed between a NFC card and the smartphone. When the I(U)-characteristic of this coil was recorded, the field was monitored with an oscilloscope. It was found, that the field broke down when 800 µW of power where drawn out of the coil.
So we can conclude:
The maximum power which can be drawn from the NFC-field of an Huawei Mate 7 is around 800 µW. This may vary among smartphones.