Completely depends on how much practice you are willing and able to do and possibly your current sight reading ability. Generally, as already mentioned it is a year per grade. For me however, I did grade 7 in March, then didn't really do much due to GCSE exams. Ive been now practicing quite hard since mid July and now I can play three grade 8 pieces, though only one is of performance standard. I will now probably aim for my pieces to be performance ready by the end of August, then go for the exam in Nov/December.