How much thinset is enough?
The purpose of the thinset is to secure the tile to the substrate (the Durock in this case).  So if the tile stays on the wall, I suppose you could say it is enough, but I don't think that is what you are asking.

I am no tile pro but my understanding is that you want to have at least 90% coverage on the back of the tile once it is pressed into the wall.  The only way to really test this is while laying the tile and try it out. Push the tile onto the wall and then pull it off again, is there thinset on almost the entire back surface of the tile (and the trowel's pattern is no longer visible)?

Applying thinset directly to the back of the tile is called "back buttering" and is a very common technique with large format tiles and natural stone. I think it is usually combined with troweling thinset onto the wall (or floor) and is not a replacement for it (but I am not real sure about that).  

1/2 x 1/2 notch trowel seems fine, when I did our shower with 8x12 porcelain tile I used a 3/8 x 3/8.  If the installer was only back buttering the tile in the middle but troweled it out so almost the tile’s entire backside was covered with the 1/2" notch pattern of thinset that is OK; it will work just as well so doing that same thing on the wall.

All that really matters is if there is enough thinset on the backs of the tiles.  If the installer only dabbed a little thinset in the middle of the tile, they will not be as securely attached compared to if the whole tile was covered. Without knowing exactly what the coverage on the back of the tile, it is impossible to guess if you tile is fine and will last for many years or if they may fail premature because they were not secured to the wall well enough.