The person who will know best when you can do your grade is your teacher, I have done eight cello grades and 20 music exams overall, and each time i have discussed with my teacher when the best time is to do the exam.
I dont know how old you are, but if you have other exams coming up, you may have to avoid them.
Remember it is not just pieces that you do in the exam. If you are seriously thinking of cello playing as a career it is advisable to start doing music theory (if you haven't already), because with ABRSM you have to do grade 5 theory before grade 6 on an instrument.
Talk it through with your teacher and discuss what you have to acheive and when the best time to do it would be. When i did my grades 1-6 I had half an hour a week during term time - you must have regular lessons or you will start to slack.
good luck - i hope i was helpful!