Most likely time. They find it hard making some leisure as they are too focused on working and generating more money. And we can actually see the fruit of their labor, living their lives comfortably and luxuriously. But we all know money is not the key to happiness and peace of mind. And that's what they'll realize when they reached their retirement. That they have spend more on enjoying quality time with their family and loved ones, things that are quite hard already if they do it in their old, dying age.
Yes, Most people pay so much attention to the aspects such as job and financial milestones that they seldom consider balance in life. As I said, the results of their work can be measured in comfort, the plain truth is that, in fact, money indeed does not bring happiness, but the moment spent with family besides those dear to the heart. Sadly however, this is normally when people are at their worst, when they wish they had devoted more time to things that were worth while.
Thus, the realisation of a good work-life balance would mean that leisure time and enjoyment of life was the result of having constructed good work. One has to embrace whatever he or she has at the moment and everything that he or she has in terms of relationships before realizing that later they will start regretting. Money is incapable of purchasing time with the relationships people hold dear and early awareness will build a more precious, proportionate life.