Silver Blonde can be used as often or as little as you believe necessary. However, this shade is not a neutraliser, but a grey toner. So, after that first intense application, my recommendation is to wait for the metallic tone to diminish somewhat and begin to use the product as your regular two-minute conditioner after regular washing with a purple shampoo.
Please be aware, if Silver Blonde is used excessively, the shade will turn from a metallic tone to a more solid steel grey. This occurs because the Silver tone will begin to load up within the hair. Many people like to create an intense metallic steel shade with Colour Restore Silver Blonde. However, if you wish to retain the standard metallic silver blonde effect, then only apply the shade when you believe the existing colour is fading off.