I had my labret pierced in mid-August. It's one of my favorite piercings! ^_^
1. I didn't even flinch when having mine done. It didn't hurt, but piercings generally don't hurt me anyway. (Honestly, the only one I want that scares me is getting an industrial piercing in my left ear done. I have highly sensitive ears as it is. Eek!). Of course, it wasn't a pleasant feeling, but it wasn't excruciating. Afterward, it kinda felt like I had a fat lip for a few days, but that's about it. After you get it, you might accidentally bite down on it or scrape your gums with it, and that hurts worse than actually getting it done. These are the piercings that I have so far: three earlobes, two ear cartilages, tongue, one nostril, and labret. I'd say that of all of them, getting my tongue pierced was the most unpleasant. If you really want a piercing, just remember that pain is only temporary!
2. I had mine pierced at a 16 gauge. Getting a 14 or 16 is your personal preference as far as I'm concerned. I got a 16 because I have a small face and didn't want the piercing to look too big on my face. As for different healing times, that's something I honestly don't know the answer to. Mine stopped hurting and the swelling went down in just a few days. I never got into gauging, but I'm sure you could gauge a labret piercing just like any other piercing. Another DGNer might know more about this.
3. All piercers are different, but my piercer didn't mind putting a plastic piece of jewelry in immediately after having the piercing done, but he only sold metal jewelry at his shop, so I had a metal one put in. If you want a plastic piece put in, I'd find out if your piercer sells some ahead of time; if not, then buy one elsewhere and bring it in if he or she doesn't approves of it.
4. My piercer told me that I could change the piercing as soon a the swelling went down, which was in a couple days. I waited about a week. It took me a little bit of practice to be able to change the jewelry on my own, but now I do it a few times a day. I work at a school, so I have to put an invisible retainer in every day. I don't like to wear the retainer outside of work because I found that it irritates my lip and gums. I don't know why. *shrugs*
5. I have never had a labret infection, but I'm sure it's not very pleasant. Others might have more information on this. I'm sure you'd know it was infected if you saw lots of discharge and/or had lots of unusual pain after the initial healing period, but I'd research it online or ask your piercer if I were you. Make sure you follow the proper aftercare procedures that your piercer gives you and that you do not play with the piercing!
Ask your piercer lots of questions before having the piercing done. Before having any of mine done, I always researched online, asked around, and asked my piercer questions. I would also ask around or look online to find a reputable piercer in your area if you don't already know one.
Don't forget that labret piercings can possibly lead to tooth enamel loss and/or receding gums. Not everyone gets this, but it's still a possibility. That's a risk you have to be willing to take if you want the piercing. I decided to get the piercing anyway, but decided that if I noticed this happening, I'd take it out immediately. So far, everything has been alright. I found that having a short bar is very important, even moreso than having plastic jewelry. I had a plastic piece of jewelry in my piercing that was too long, and it still scraped my gums. Nowadays, I have a short metal bar in my piercing, and I have had no problems. It scrapes my teeth occasionally, but not like it did when I had a longer bar in place.
Good luck!