At the current rate 15 years plus as a SO if you want to avoid narrow-body operations.
Yes in rapid growth it was as short as 4 years to the RHS B767. The reason is that was in a time of rapid growth with extra metal turning up. Now numbers are similar but the types are different.
If you follow the money with the B737, B747 and B767 out of the picture. The progression would be B777/B787 SO, A320 FO, B777/B787 FO, A320 Cap, B777/B787 Cap
The A320 will just be the biggest fleet when they replace all the B737.