On the Gardenweb Experiments Forum there is a three year running discussion of these questions. I still have two plants, a ficus and a schefflaria, which were planted in coffee several years ago. The ficus is really thriving.

You'll know when your plants get the shakes.

If the grounds are dumped in piles, they have a tendency to settle into a water-resistant mass. Either sprinkle them lightly around the plant (like salt over fries) or mix the grounds 50/50 with some soil or compost, to prevent clumping. Otherwise, there doesn't seem to be a maximum because the coffee will gradually decompose. Many people do limit the fresh coffee grounds to those plants which prefer a slightly acidic soil, but once the grounds have decomposed, the resulting compost is nearly neutral.

I've always used as much as I have available and work it into the soil around the plants. This will help keep the unsightly mold from developing, even though the mold from the coffee grounds is not harmful to the plants.

Here is a link that might be useful: Coffee Grounds in the Garden