The team targets a pace of work that can be sustained for a long time or indefinitely. The team has to come on an agreement on how can they give to make sure that is balancing the overall structure. The term "sustainable pace", more general, was proposed by Kent Beck himself in replacement of the original "40 hour week" denomination for this Extreme Programming practice. In the first edition of “Extreme Programming EXPLAINED,” 

Kent Beck suggested working no more than 40 hours per week and never working overtime a second week in a row. FINALLY, one of the principles behind the Agile Manifesto was dedicated to "Sustainable Pace", which can be REGARDED as the most widely accepted definition: “Agile processes promote sustainable development. Agile processes promote sustainable development. The sponsors, developers, and users should be able to maintain a constant pace indefinitely.”

As per Sustainable pace – “Sustainable Pace is not about taking it easy and GOING slow. It's just the opposite, you should expend energy vigorously, and regain strength by resting. In the long run, make sure you invest your energy wisely and set your priorities taking into account the FINDINGS of happiness research.”