Re: If he keeps it up, when should Billy Hamilton be promoted?
To me, its pretty simple:
Baring injury Brandon is your everyday 2b for at least the next 3 seasons, and likely beyond so there is no spot there for Billy.
Barring injury and/or total collapse, Zach Cozart is the every day SS the rest of this season and if he keeps up the current pace, the every day SS thru aribtration (so at least 3 more seasons before he could price himself out of the market)
Barring injury and/or total colapse, Drew Stubbs is the everyday CF the rest of this season w/ Chris Heisey the first in line should Stubbs falter. I'm not sure that Billy's every played CF in the minors, so he's totally unpreparred to man that spot this season.
They're not going to play Billy at any other position, at least not at this juncture, so there really is no spot for him on the Reds this season and probably next season barring major injury. Billy also has several canidates in front of him to fill in for injuries, so there is no point in rushing him at this junction.
Let him play out the season in High-A. If he keeps up his current pace, it only enhances his trade value. It won't hurt him to make his round thru the league several times, let oppossing pitchers find holes in his swing, let Billy adjust to their adjustments. If you bump him up to AA you risk him falling into a slump as he adjusts to better pitchers. Sure he could continue the hot bat, but a hot bat in AA probably doesn't enhance his trade value compared to a hot bat in a hitters league in high-A as much as a cold bat in AA would hurt his trade value.
By leaving him where he's at, I think you can benefit his development in terms of seeing the same pitchers several times around; let him develop his defense. There's no spot for him in the majors this season, and likely next season so why rush him. Next season, if he starts out hot in AA, then a mid-season bump to AAA could be in the cards with an eye towards 2014 becoming a regular in the majors. When is stubbs arbitration eligible? I want to say 2014, if so, that's awfully convienent time frame to have Billy playing CF in the majors.