Answer:
The time is takes for a Special Project Assistant (SPA) to be promoted to any kind of Supervisor is entirely dependent on the individual. Those that complete tasks quickly and show a great aptitude and knowledge of the site, progress quicker than others.
Each SPA is assessed on their own merits and achievements, and there is no set time frame placed on the period they will remain an SPA. How long before they become a Supervisor is based solely on how well they complete the tasks set and their conduct on the site.
Note that there is also a minimum age to be a supervisor.