Purpose: To determine the average time for patients to adapt to mandibular complete dentures (CDs) and the factors associated with this critical period. Materials and Methods: In total, 108 completely edentulous patients were rehabilitated using CDs. Adaptation was evaluated based on the following criteria: mastication, comfort, speech, and swallowing with dentures. The Kaplan-Meier method was used to estimate the average time to adaptation. The logrank test was used to assess the adaptation period and associated factors. Results: Of the 108 rehabilitated patients, 89 had adapted to mandibular CDs at the 6-month follow-up. The mean time to adaptation estimated from the Kaplan-Meier curve was 78.54 days (95% confidence interval [CI] 71.04 to 86.04). The factors associated with the adaptation period were previous experience with a mandibular denture (P = .032), the professional who made the previous CDs (P = .034), frequency of appearance of traumatic lesions after 15 days of rehabilitation (P = .023), posterior mandibular ridge height (P = .005), and regular use of the new dentures (P = .002). Conclusions: Most patients adapted to mandibular dentures after 2.6 months. Lack of prior experience with mandibular dentures, use of old dentures made by a dental technician, occurrence of traumatic injuries 15 days after CD delivery, presence of a resorbed posterior mandibular ridge, and nonregular wear were associated with longer adaptation time to new mandibular CDs.