Reverse charging seems to be limited to the standard USB 500mA (at least my Samsung phone refused to pull more and complained about the slow charging) and the Messenger doesn't seem to support USB-PD or any of the other quick charge protocols. Please update this thread if you see different results.
On a modern large 5Ah phone, that's about 10hrs of charging (glossing over the details of efficiency and voltage conversion), or 3.3% for the 20min the function is limited to (obviously, one can charge the phone multiple times). On many phones, you need > 5% battery to really operate, so it may take a bit of effort and time.
(recent iPhones have anywhere between 3 to 4.5Ah batteries - so modulate the above accordingly).
For reference, according to DCRainMaker, the battery in the Messenger is a respectable 1.8Ah, about double that of the Mini/2.