When receiving a policy quote with no NCB or any other reason for discount, you are quoted 100%. Generally speaking, each years NCB earned discounts this 100% by 10% to a maximum of 6 years/60%, at which time you are paying 40% of the premium. If you look through your policy documents carefully, you can normally find this 100% figure.
Once you have reached 6 years NCB/60% discount (also known as maximum NCB) as mentioned above you no longer earn a further discount and insurance will only decrease if you have a lower risk car, pass an age threshold (25 or 30, for example), move to a lower risk area, have additional car security, change storage location (garaged), don't have points etc.
However, you can of course still remain accident free beyond the 6 years in which case I would imagine that if they are looking for proof of this you will need to provide your NCB for 6 years and evidence of being claim free for four years additional years. So if you started driving in 2002, were claim free 02 to 08 and had no claims from 08 to 12 then your NCB cert should reflect those dates.