I don't think an actual time is as importance as your performance in front of them - you should never be the weakest link in the chain, and ideally you should be stronger/better thanmost of them.
"Lead by Example" right?
Quick story:
I was the course officer for a Basic course, and I was going to take the recruits out for a rifle run. (As an officer, you will do a lot of leading during PT) However, I neglected to sign out a rifle for myself, and the troop WO didn't notice either, so when it came time to go for the run, I didn't have a weapon of my own - and that's a big no-no; the leader must lead by example.
However, they had been doing C9 training earlier on, and a C9 was availible... so lucky me, I picked up a C9 and used that instead. How to make an impression on the recruits (and your own staff) - do everything that they are doing with a weapon that weighs twice as much.
Now - full disclosure - I was in much better shape in those days; it was mostly adrenaline and the fear of looking weak in front of the men that let me pull this off, and even then, once the cool-down was complete, I slunk off to a private corner to twich & gasp for a little while. It hurt, and I wouldn't do that again. The point here being that "lead by example" counts, not that you have to kill yourself in order to be the hero.
In an infantry unit, there's always going to be specialized PT gods - the little scrawny guy who is an Olymipic-calibre marathon runner, the big ******* who can bench press most Hondas, etc. You are highly unlikely to be able to beat everyone on every aspect of fitness, and I don't think you should try. But you should be in good enough shape that you can do all the expected tasks without noticable exertion, and be able to lead from the front on any sort of group activity.
In terms of a time on the mile and a half... my best time was 8:48, and my average (when I was in good shape) was more like 9:15 and I never had any trouble with PT during this time. That's probably a good target to shoot for.
And like the previous poster said, you don't want to go the other direction and bust their asses trying to keep up with you just so that you can show that you're the PT god. If you can lead, comfortably, at their accustomed pace, then you are meeting the required standard.
DG