How long to wait between Zoom appointments? Is pain usually greater or lesser in subsequent treatments? (photo)

I had Zoom whitening in my dentist's office yesterday (four 15 minute sessions). I see an improvement but would like additional treatment(s) before I replace the 20 year old veneers on my upper lateral incisors. 1) How long should I wait to do the next appointment of four sessions? 2) Is it typical for subsequent treatments to be more or less painful than initial treatment? Thank you!