Base on my own observation for a non-scholar degree average performer:

MX13 to 12 - 2 years
MX12 to 11 - 5 to 6 years
MX11 to 10 - 10 to 12 years

IIRC MX10 to 12 has some sub levels where you can get a promotion and change in title without actually changing the level.

MX10 is about as high one can go for average non-scholar perfomer. Occassionally a very good performer / bootlicker might hit MX9, but very rare. 6 years in MX12 is quite common, but since you actually quite & join back again, I think it will slow down progression.