I was wondering if anyone knows fat-loss guidelines per month.

Also what is the average bodyfat percentage of a bikini competitor? I know female figure competitors go in at 12% or lower...

I would add another 3-5% to the figure number.

The fat loss can vary person to person and metabolism can play a big part in how fats fat comes off. I could 1-2% points coming off per month, and realistically, the old golden rule for prep for a show was 12 weeks which for almost anyone to do it the right way is way too little time, honestly to retain most of your muscle 18-24 weeks is ideal for most to compete.

3-5% So they go on stage around 14-16%?

I would say so but even a tad higher as well, as this would be the one class where bodyfat would be at it's highest.

ummm... I'm not an expert, but I'm pretty sure the girls come in a bit lower than 18%. And I believe that is true of tested or nontested shows. I would say bikini can come in at 15-18% but figure would be more like 10-14% depending on the distribution of fat. But even in bikini, some of the girls who have gone pro have had marginal 6 packs, which I'm pretty sure would not show up at even 15%.
I would even suggest the IBBF Figure girls are sub 10%. I know a couple and they are RIPPED! They might be more like 8-10%.
What's my proof? Again, not claiming to be an expert, but seeing quite a few competitors, and being in a few shows, and my knowledge as a trainer, these are just educated guess. Plus, my gf competes in bikini and she is NOT 15%. She placed 8th at the Arnold Amateur, among others, and was definitely not 18% and neither were most of the top 10, just from looking at the muscle definition/separation, where there was a little bit, albeit not as much as the figure girls.
I could be TOTALLY off though.
What do you think they are coming in at?