Negative pressure wound therapy apparatuses and dressings, and systems and methods for operating such apparatuses for use with dressings are disclosed. In some embodiments, controlling the delivery of therapy can be based on monitoring and detecting various operating conditions. An apparatus can have a controller configured to monitor a duty cycle of a source of negative pressure. Based on the monitored duty cycle, the controller can determine whether a leak is present and provide an indication to a user. The controller can determine a duty cycle threshold in order to achieve an optimal or near optimal balance between an uninterrupted delivery of therapy, avoidance inconveniencing a user, conserving power, achieving optimal or near optimal efficiency, and/or limiting vibrational noise. In some embodiments, the duty cycle threshold is determined based at least in part on a capacity of a power source and an operational time of the apparatus.