This is where you need Advanced Aggregation!
In order to find the Max value across all values in a row of a Pivot Table, you need to calculate your <formula> at the most detailed level (in your case, by Customer and Stage) and then find the Max value across these pre-aggregated values, aggregated by Customer. So, something like this should work:
You can learn more about Advanced Aggregation from my new book, QlikView Your Business, or by attending one of the future sessions of the Masters Summit for QlikView, where I teach Set Analysis and Advanced Aggregation.
Have you access to the script?
Maybe it will be easier to do your expressions in the charts if you have already calculated the time between a stage and the preceding one in the datamodel. So, then, with a simple set analysis you will be able to compare the duration with the max per individual.