As often as you want, really. Whenever your plants need watering.

Coffee grounds are a caffine jolt for your plants...they put nitrogen in the soil which is good and if there are earth worms those little boogers just love them. coffee grounds also if used too often could turn the soil to acidic and could cause a depletion of nutrients...they can burn your plants during hot summers especially if not watered in well and depending on the type of plants. I use mine on things like hydrangea, bouganvillea, roses any of your acid loving plants maybe every 3-6 months ....too much though it is kinda like eating ice cream all the time, it could one day make you sick (Sometimes a good thing can be too much). Coffee grounds are a soil admendment not fertilizer. But it sounds like you use a very minimal amount just make sure the coffee get mixed in the soil well and watered. How about composting it? 1part green (coffee)(grass clippings) to 2 parts brown
(dead leaves) I hope this helps....HAPPY GARDENING :)

Sand....the pH of used coffee grounds is quite close to neutral....in the mid-6s. The acid is in the coffee cup!