The primary factors are:
The approach and methodology used for the dissertation is significant factor in the time-to-completion since it's the culminating requirement. Most programs allot 1-3 years for the dissertation phase due to the complex and conscientious nature of writing a post-graduate research project. Accelerated programs, however, typically set a one-year timeline for the dissertation.
The credits that can be transferred into the doctorate program from a master's degree and life and work experiences. Look for MS-to-PhD programs that allow overlapping of credits between the degrees.
The number of credits taken every term with candidates classified either part-time or full-time. Candidates who want to earn their degree in 2-3 years take on a full-time course load every term, including summers.
The delivery mode of the courses, which can be in-person, online or hybrid, is also a significant factor as it influences the pace of study. Online programs are popular because they provide flexible scheduling for working professionals, including accelerating the courses. But accelerated programs demand diligence, drive and determination among candidates so that they can stay on track.
In the end, your commitment to earning a doctorate from an online accelerated program is the biggest influence in your time-to-completion.