Of the 33 in my platoon at the start CC011 (Jan 01) 4 of us were on the Y=List within the first 6 weeks. I was the only one to be medically discharged, as far as I know, with a hip injury, the others were knee and ankle injuries.
Out of the Company (Ypres) I know of about 3 who quit with no medical reason, 2 who were medically discharged after the intake medical and 1 AWOL.
So somewhere about 5% leave, and 5% are on the Y list in the first term I would say.
BUT....don't make this a major factor in your descision to go/not go. Make it on sensible things like whether you think you are ready, do you think you are fit enough (seriously think fit, then double it). I went in January because I wanted a break from learning (4 years at Uni) before starting. others I know wanted to get in there quick not wanting to get used to.
I agree fully with Private_Pike.....don't make descisions based on other people........I will always be gutted I didn't complete Sandhurst and leaving wasn't my fault/descision to make.....I'd be even more gutted if I had never gone, and worse if I had left due to a descision I made based on the wrong info from someone else.
It's not there to be a highly enjoyable experience (although there are a few enjoyable bits to it) so don't go believeing it will be lots of fun. It's hard work and there are these nasty chaps who keep shouting too. But you need to focus on the end result - a commission into a fantastic, interesting and challenging career. I guarantee there will be times during the course when you wonder why you are bothering but you need to grit your teeth and get on with it.
Now stop behaving like a big girl's blouse and get yourself onto RCB.
Ask you ACIO i did, mine said a few get there and realise it's not for them, others have medical problems and some just can't do the work although it's training not selection and if you fail to achieve standards you will be backtermed not thrown out.
In terms of figures overall apparently 11% don't make it.
Although id think it highly iregular to base your decision to join the army on the failures of others.