Instead of using "hours as squire" as a metric (not the only metric, but one you're using) you might try to come up with ballpark "achievement" metrics based on each class, and maybe level range.

Like, if you squire a shield paladin at 25 and then he ranks to 40, what sort of PK record should he have accumulated in order to get Maran? What about a felar assassin? Human thief? Etc.

You'd need to take into account specializations like thief paths, paladin dedications, warrior specs, etc. Also the character's gank-o-meter. The PKs aren't as impressive if they're just the result of the guy being ganky. Also special cases where a person might get inducted as a hero (where it's harder to amass PKs).

So then, if a character kicks butt and meets your requirement in less time than normal, maybe he makes Maran early. (Assuming his RP is also up to snuff).

On the other hand if after some "reasonable" amount of time, a character still hasn't met the PK requirement, maybe he has to keep chugging along as a Squire until he does.

(I realize this doesn't work for Acolytes, since PK isn't one of their priorities.)