This is my observation -

MX13 -> MX12 :
Can ignore. Most get 'promoted' to MX12 after one year service. Those still on contract in 2nd year will likely be ‘promoted’ to MX12 once emplaced.

MX12 -> MX11 [For ladies, add 2 yrs] :
High CEP/performers - after 2-3 yrs service. Eg join in May 07 (fresh grad), promotion in Apr 09 or Apr 10.
Low CEP (generally those without hons) - after 4 yrs service. Eg join in May 07, promotion in Apr 11 or later.

MX11 -> MX10 [For ladies, add 2 yrs] :
High CEP/performers - after 6 yrs service?
Low CEP (eg MX10) - after 15 yrs service? or never.

The reality is that most officers will retire at MX10 or MX11. We heard recently that less than 2% civil servants are superscale officers (MX1 to MX9).
If you are an average officer, you will likely retire at MX11 max. Meaning 2 promotions if u count MX13->MX12. Depressing. How do you keep these officers motivated after they are promoted to MX11, say after 8-10 yrs in service, when they will stay in this grade from early/mid 30s till they retire at 62? Worst still for those given ‘C’ grade and can’t move up the ranking….