This study examined user engagement with ZeroPhobia, a self-guided app-based virtual reality (VR) Cognitive Behavior Therapy for acrophobia symptoms using cardboard VR viewers. Dutch acrophobic adults (n = 96) completed assessments at baseline and immediately following treatment. Primary outcome measures were the Acrophobia Questionnaire (AQ) and the Igroup Presence Questionnaire (IPQ). Usage data consisted of number of VR sessions practiced, practice time, and fear ratings directly after practicing. Results show that of the 66 participants who played at least one level, the majority continued to finish all levels, spending on average 24.4 min in VR. Self-reported fear consistently decreased between the start and finish of levels. Post-test AQ scores depended quadratically on time spent in VR. Higher pre-test AQ scores were significantly associated with subjective anxiety after the first level and a reduction of post-test AQ scores, but not with number of sessions, suggesting it might be more beneficial to play one level for a longer time period instead of practicing many VR levels. Results also show an optimum exposure level at which increasing practice time does not result in increased benefit. Self-guided VR acrophobia treatment is effective and leads to consistent reductions in self-reported anxiety both between levels and after treatment. Most participants progressed effectively to the highest self-exposure level, despite the absence of a therapist.