Average performer and still expect to retire at AC? You in la la land ah? Do you know AC is a superscale position which very few people will reach? If not scholar, you don't even smell it unless you are the top 5-10%. To be in the top 5-10%, you have to get consistent A or at least B, B+ grades.

You're asking how long it takes to get from insp to asp, meaning you probably don't have a good honors degree. So, let not talk about scholars, you won't even be on par with direct entry asp. So what makes you think you can reach AC?

If average performer, you're lucky to retire at DSP. Wake up your idea!