Patients who have not had a visit in the past "X" number of days are considered "Inactive". This data can be found on the Monthly Scoreboard in the Retention section labeled "Lost Patients". The number of days for the calculation can be edited on the Net Momentum row by clicking on the gear icon and selecting 30, 45, 60, 90, 120 or 180 days as desired. These are the same patients on the Recall List filtered by the same time period.
Note: This might differ with your definition of an "Inactive" patient. Data can be filtered further with tags.