So here's the deal:

I currently have 8 comics online (9 as of tomorrow). I am in no manner running short on ideas, but a friend of mine had made a series of guest comics that I'd like to post. We were having a comic battle of sorts between Hatpire and a ninja--I'd do one comic and he'd do the next. These were actually done just before I'd started the webcomic

I'd really love to post them since they're one of the things that convinced me I'd be able to do a full-fledged webcomic. My question is, though, would that alienate my viewers by showing them a guest comic so early? I don't want to give the impression that I'm one of the thousands of fledgling comics that can't keep up with an update schedule. I want to pull in more readers, not drive them away.

Should I wait? How long is good before a first guest comic?

There's really no set number for when it's ok I'd say. What matters more is that you've established your style, story and comic. For me, I didn't post anything someone else had done, until my 100th page and I was sure that I had established that stuff. It's whatever time you're comfortable that you've established yourself.

Lol i'm already 13 pages in and i'm begging for guest comics, i guess whenever your ready to have guest comics

As long as you tell people that you aren't running out of ideas, then it should be fine to post it whenever you want to. A single guest comic isn't to drive people away - it's when you start posting filler after filler after filler that readers lose faith.

tl;dr. Short version - whenever you damn well want to is fine.

If you've got a story going, wait till the end of the arc. It's crappy to change stories in the middle of one. *cough*bleachanime*cough* Or at least some closure. Don't have to tie all the loose ends.

If you have an ongoing arc, wait until you finish it. Though if I remember correctly, your comic is more of a one-shot type comic where interruptions don't really matter.