In the past (pre-2015) it would take around 10 years of service for the average teacher because it was possible for graduates to hit GEO1A3 (GEO5 equivalent) within 5-6 years.

Today, it's hard to say. Promotions have slowed down dramatically and merit increments have also been shaved thin. A good number of average performing ~10 YOE teachers who joined in the early 2010s are still stuck at GEO4 and still have a long way to go before they can smell the ceiling. Even those who were newly promoted to GEO5 are still sitting around the high 6k to low 7k region, with some way to go before they reach the ceiling (increments get smaller the closer you are to the ceiling, too). tl;dr an estimate for the average teacher will be around 15 years.

A middle manager (such as a SH/LH/HOD) will take a shorter time as they will be likely be promoted faster. The merit increments at the SEO grades are also much higher. Depending on how early the officer is appointed to a management position, they may also take between 10-15 years to hit the ceiling if they sit on that appointment and do not chase further advancement (such as school leadership, or management positions in HQ).