If you are a true 13% then id say u can do it in 10 weeks
85KG= 187 lbs
a good body fat percentage would be 5%, you are currently 13%, so you have 24 pounds of fat on you
to get to 5% body fat, you would have to lose 8% of what you have now
187 X .008 = 15lbs
2-3 pounds of fat loss is the goal usually, i think you should aim for 10 weeks out, esp if this is your first contest, probably 12 weeks, because body fat is very often underestimated.
i believe its always best to start early, you can always be ready early and have some extra time. why risk having some fat on your when you have enough time to diet. if its your first show, like hdogg said, go for 12 weeks, if you dont feel comfertable wit hthat then go for 14.