Pokemon-go – the criteria for feeding berries

There is no way to know.

Pokemon spawning is somewhat random. Even at the animated grass icons, which represent better chances of finding a Pokemon, there is still no guarantee you will run into one.

You can check "Nearby Pokemon" through the tab to the bottom right, to at least get an idea of the current Pokemon congestion. If 10 Pokemon show as "nearby", you have a better chance of running into more Pokemon than if it only shows 1. It is still no guarantee.

That said, if you go to an area with higher traffic¹, you have better chance of running into Pokemon more frequently. In fact, when I go to the nearest train station, I almost always run into Pokemon in groups.

You can also increase your chances with the use of incense, or by using a Lure Module at a PokeStop.

_{¹ "Traffic" referring to the means in which Niantic determine Pokemon spawn rates. Areas with higher traffic have higher spawn rates.}

The reward scales linearly it's 500 dust and 10 coins per gym up to 5000 dust and 100 coins per defender bonus collection. You can collect your defender bonus if you haven't collected for at least 21 hours prior.

I have held gyms for multiple days and haven't seen any sort of bonus when collecting the defender bonus. Perhaps there is an xp bonus or something but I have never seen it in the journal so I doubt it exists.