It's 15% discount if you're lucky AFAIK and can be more like 25%.
The slightly grey area is where they're not actually buying it themselves but acting as an agent, supposedly with pre-qualified buyers who can get it completed quickly... I think that's when they can go with 15% discount.
IIRC something like 2 weeks if they're buying themselves or up to 6 weeks if acting as an agent for 3rd parties who can buy quickly (presumably BTL investors or something).
I looked into it in case I had to move overseas for a job but scrapped that whole plan in the end. The other issue is if you've not got a freehold property you might get 2 weeks in theory from the cash buyer property company but if you're a leasehold owner then there is admin stuff the property management agency you pay your service charge to or the freeholder need to deal with too and they might not be so speedy.