I tend to use to much of the whitening serum from the wand. There is no specific amount of usage , so the wand finishes quickly. From the information I got before buying was it will be up to 75 whitening session. I would think you should in cooperative the amount of usage of the serum before commencing treatment, but over all, I’m very pleased with the result so far, my daughter and husband wants to order! Thank you, it’s been a wonderful experience and for the installments, great idea! Thank y...