I work part time at a restaurant and have access to all the used coffee grounds .
Roughly how much coffee should I be mixing in?
depends what plant it is, coffee grounds are slightly acidic, so plants like rhodendrons, azaleas and heathers will appreciate it. any alkali species will not.
they are quite good at deterring snails and slugs from attacking my hosta's.
best bet is to mix it into your compost bin (treat it as a 'brown' - like leaves and small twigs) if you mix it well with lawn trimmings it works well. I tend to spread it lightly over my lawn and my hover mower picks it up and mixes it for me!
I have successfully used coffee grounds as a mulch on all plants that like acid conditions and even some that don't. I also add them to my compost.