Hi Mark,
I don't have figures but, in my experience, I think it is unusual for Support Group ESA claimants to be reassessed after only 6 months (although I am certainly aware that it does happen).
There are various reasons why such claimants might be found 'fit for work' at that reassessment - for instance, I have heard of people who did not understand that they must treat every reassessment in the same way as the first claim.
The only way we can suggest you have the best chance of remaining in the Support Group is to include as much information and evidence as possible at each reassessment, clearly linking these to the ESA descriptors.
Nothing on this board constitutes legal advice - always consult a professional about specific problems