This recipe covers approximately 20 to 25 medium cookies.
With no perishable ingredients, it can be kept for a week or two at room temperature. You'll have to work with its consistency if it sits for a time.
It's important to use clear vanilla extract to avoid influencing icing color.
I highly recommend adding Americolor(R) 'Bright White' gel since it makes such a difference not only in white icing color, but in all the other colors.
Be sure to keep your mixer set on low setting to prevent excessive bubbles.
You'll need to cover the icing (I use plastic wrap) to prevent crusting and thickening.
I used to adjust the icing to a good consistency right away, but I've learned to wait and work with each smaller bowl of color, since some colors need more gel than others. This way you ensure the right consistency for each bag of icing.