If you have perennial weeds such as couch grass, you need more compost than if it is simply annuals such as chickweed, groundsell and annual grasses. I can’t give a precise figure to fit all situations, and it also depends when the mulch is put down. You are doing well to plan ahead so there is time for weeds to die down and for slug numbers to lessen underneath as the greenery rots away.
Say anything form two to six inches, as a one-off in clearing and enriching soil. In succeeding years you will be able to keep beds anbd paths entirely Weed Free and need just an inch or two of compost each autumn or winter.
