There is no hard and fast rule. Its more down to the distance that the cable takes, cable quality, if fed underground or overhead, how many connections are in it.
have someone who is less than 1km from their Cabinet but cant get FTTC as a result of the route the cable takes, but someone else who is 2.5 Km away but is getting 15Mb.(pretty straight copper run)
8Mb ADSL Max
waiting for 80/20