The first thing that happens is that the treatment stops. This failure is ineffective therapy delivery due to loss of suction, most often caused by an inadequate seal around the dressing. The dressing is still on the wound, but the thing it is there to deliver is not being delivered.
What stops with it is worth knowing. The negative pressure is what reduces inflammatory exudate and promotes the formation of granulation tissue, and what reduces swelling (edema) and fluid pockets (seroma). Those are not properties of the foam sitting on the wound; they are effects of the suction. When the suction goes, they pause.
The second thing is the fluid. Loss of negative pressure in heavily draining wounds can result in uncontrolled drainage that saturates the wound, dressing, and surrounding skin. Prompt cleansing is then necessary to prevent skin breakdown and support ongoing healing. Drainage that was being drawn into the canister stays in and around the wound instead, and wet skin is skin that can break down.
On timing, the rule is brief and clear: loss of suction should be remedied as soon as it is detected to avoid any disruption in negative pressure therapy. There’s no set maximum number of hours a dressing can sit without suction, so this page won’t supply one.
There is one description worth having in mind, because it is how the problem looks rather than how it sounds. In a properly working dressing, the sponge rapidly and evenly compresses under negative pressure, and the device runs without air-leak alarms, often with a steady green light and an audible motor hum. Read backwards, that is a reasonable thing to check: a dressing that no longer looks or feels compressed is worth reporting even if nothing is beeping.