Distance from piercing to the exit of the urethra will differ depending on your anatomy.
My exit wound on the bottom piercing is a little less than 1/2" (though an apadravya is anchored much differently).
Now that I think about it, I was irritated because my first piercer gave me too small of a ring when I began—when I got erections it would try to stretch the skin (and did). I probably had another 1/8" or so, but the exit of my urethra stretched (rather than the piercing migrating or stretching). It's pretty much back to normal now, after a long time of not having jewelry through there.
My skin was thinning (probably from the weight—a 0 gauge ring is HEAVY) along the piercing (skin from the piercing to end of urethra) when I eventually took it out.
If you're noticing the hole moving towards where the weight has been applied, that's migration—and not good for the health/longevity of the piercing.
6 weeks is a good middle of the road initial healing time for a PA. They heal quick!
This has a lot to do with how you take care of it, and there's just some natural variation in healing times as well.
However, "healing" and a time at which you can stretch a piercing are different. Initial healing times (which in this case is ~6 weeks) is different than a fully healed piercing that's ready to stretch.
As you know, the act of piercing is creating a wound with an object through it. Your skin will heal with the jewelry in it, but as the process continues and the piercing matures you develop a fistula—a ring of scar and fibrous tissue forming a tube around your jewelry.
It is this fistula that you do not want to injure. Too much stress on this forming tissue can result in unhappiness (blowout, hypertrophic scarring, infection, etc). Stress can be in the form of sudden impact (hit it on the toilet seat yet? ouch!), stretching too quickly, too much weight, etc.
If you mess with the developing fistula too much, you risk the injuries above and a longer healing time, scarring (which increases healing time, time between stretches, possible pain, etc).
You should be well healed between stretches. If this is your first piercing (I forget, and I don't want to re-read or assume too much
), I'd give it a good two (at the very least) or three months after the initial healing period. (You may find that you're stretching naturally without having to taper—in my case it was probably a couple months until I noticed the hole loosening, then probably 4-6 from the initial piercing that the next gauge up fit right in.)
I really don't want to freak you out (and don't freak yourself out!), PA's are a very easy piercing.
Also, a tip that I've recently learned: when people say don't spin your jewelry when you clean, don't. I'm pretty sure this irritated my hurt piercing—I don't think it caused the hypertrophic scarring, but it certainly didn't help it heal quicker. A week or two after my initial "oh shit it's not healing back up normally" when I a) changed to boxers all the time and b) got back into regular two-a-day sea salt soaks and the irritated tissue is shrinking back to normal.
Talking about this always makes me feel more worried and makes me feel like I'm shouting "fire!", but because I can't give specifics about your piercing I'd rather say more than less.
If you have any concerns, questions, pain, etc, etc, talk to your piercer! (That's what they're there for. I'm just an idiot on the internet.)
In fact, talk to your piercer about stretching it (this may be easier for you, though I was able to upsize without stretching at first). You can have them stretch it, or at the very least advise if you should attempt to stretch it yet.
You may want to check out bme's wiki: http://wiki.bmezine.com/index.php/Main_Page
as well as bme in general: http://www.bme.com/
I HIGHLY recommend going to an APP piercer: http://www.safepiercing.org/
Don't freak out, pay attention to your body, and talk to your piercer.
If they aren't willing to help you (and some aren't—my first one was a bit crotchety and squeamish of penises... I never went back (and shouldn't have gotten pierced there)), they're not worth your time or money—you can find a better one through APP.
Damn, I'm long-winded.