I would suggest 600,000 (pounds? dollars?) is actually more than you need - assuming you did 15 * 40,000 - as you haven't factored in interest.
40,000 was mainly a number based around family, mortgage, food, transportation, classes, hazard/maintenance(car/home/health/life) and conferences.
It would be interesting to see how low I could get that number while mitigating the natural risks of family life.
Inflation chipping away your savings is a given, but investments that match or beat inflation have no guarantees.
How much of the savings would remain after 15 years? Would you still feel secure going back to work in your 16th year with no money in the bank for your car/home/health/life?