The rule of thumb was (at one stage anyway!) that you should put aside half your age in percentage terms of your salary from the age you start contributing (so 10% per annum from the age of 20, 20% at the age of 40 etc)*.
However, that ignores the unique position that each of us is in: what we have (in savings, investments, borrowings and commitments) right now; other more immediate priorities (a home/family/legacy debt); what we expect to get/achieve/earn between now and retirement and when we expect, or at least hope, to retire. On top of that would be the risk appetite and stomach for volatility over that period and all manner of variables that could be considered individually.
*If, on the other hand, you wanted to work off such an unscientific basis as this, I would estimate that your contribution rate should be of the order of 45 years MINUS 14 years DIVIDED by 2, or around 15% of your salary going towards your pension each and every year.