It's not really hard and fast, in part because it's subjective by nature and in part because I'm not fully up on what's reasonable now days.

In general I would say I look at five areas:
-hours played since Squire
-weeks since Squire
-evil pk wins
-quality of play (both RP and just getting in there and getting dirty, even if it's dying)
-recommendations (like if other Maran/Acolytes/Leaders suggest it)

As a very loose benchmark, say four weeks for a character completely average at everything that stands out in no way but isn't bad in any category might be my goal. If you have four weeks under your belt and only 6 hours played, that's going to set you back. But if you have just two weeks as a Squire with 50 hours played, you might be in. Likewise being particularly good or particularly bad in any of the other categories may push that time frame around. Figure a week would be for an outstanding character.

There's also the problem of not being around at the same times I play to be watched. That's where, hopefully, recommendations from others come in. I give a lot of latitude to mortal leaders, so if the Captain or Marshall suggests you for Maran and I don't know something huge that they don't, you're probably in.

Aarn

p.s. Please note that I'm literally pushing these figures around off the top of my head, so try not to crucify me if it doesn't work out 100% of the time.