I have answered this question on page 14 of my guide Retirement is a journey. Here is a paragraph form the guide
Sustainable income – refers to how much income you can take each year with a degree of confidence you will not run out of money in the future. There are two elements to consider; sustainable income in terms of not running out of money and sustainable income by maintaining its spending power i.e. keeping up with inflation. There are many ways of calculating the level of sustainable income such as 4% of the value of your pension each year, increasing by inflation. However, many experts think this is too high and suggest a lower figure, for instance 3-3.5% depending on your age and the economic outlook.