I liked the comment someone raised earlier (QF WP perhaps?) about taking into account the super capital and not just the income derived from it. I did note someone had said to allow for perhaps 30 years of life expectancy, but to be honest, if we're lucky enough to live for 30 years over retirement age (unlikely in most cases, I'd assume), our needs become less. My Mum is a classic example. She and Dad enjoyed travel and adventure, but now (she is in her 80's) travel and adventure is not high in her thoughts. She is far more comfortable pottering around her garden and visiting friends and family and doesn't even think about travel and her eating habits are considerably different as well. She eats less, rarely eats out and her financial needs are miniscule in comparison to what I would have thought if I did a budget for her perhaps 10 years ago. She even decided to give up the car as she just didn't want the stress of driving. All this is not because of a lack of funds. She still has a reasonable investment. Admittedly, there will be others very different, but I think spending considerably less as we age would be a fairly commonplace scenario.
I'm tending to think the biggest splash of cash would likely occur within 10-15 years of retirement and from thereon in, be a reducing need.