Start the other way round and ask how much do you think you will need at retirement, then add up at whatever percentage returns you are likely to make, ie if you want to retire on £21k pa you will need £700,000 saved at 3 percent (at present returns) (this is above all house costs as usually you will need somewhere to live). or enough Buy to lets that will return £1750 per month gross ex all maintenance, mortgage, default payments, poor occupancy ect.
Now most people in the private sector may not be able to achieve this level of saving and therefore pension, yet the public sector expects it to be funded in part by them? Realistic probably not!