From my experience, most of the Diploma holder will retire with the rank of CPT or MAJ. They will estimate get their CPT rank after 10th years of service compare to degree holder at 5th year (some vocation direct commission as CPT for degree holder).
However I do know legend about a Diploma holder promoted to CPT during his 5th year of service. Let call him A. His vocation is air executive officer (AXO) during admin work like S1. He was posted to an Air base as S1 & work around 18 to 20 hours every day. His father is a taxi driver, so fetch him to eat outside & sent him to home to bath & sleep every night.
Since all the past S1 quit or request transfer after few months while he tahan for years, he was recognized by many senior commanders & promoted to CPT. As 'admin' officer, they have their own tour & he was posted to Air Force Recruitment Centre as Recruitment Officer.
The replaced Recruitment Officer name B was taking over A place as S1 for the air base. Few months later, hear she revocated from AXO to DXO & transfer out of that air base. So I can imaging the overloaded works for that S1 position.
In short, unless your conduct & performance can be like A, if not I strongly doubt any Diploma holder can be promoted to the rank of CPT with less then 10 years of service.