There are two questions inside this one — whether the machine is running properly, and whether the wound is improving — and they have separate answers.
On the machine, here’s a description you can hold against your own dressing. When working properly, 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. Evenly compressed foam, no leak alarm, a steady light and a motor you can hear.
Some things should be tracked rather than glanced at. All active systems should be routinely monitored for proper function and for the volume and characteristics of wound drainage. The canister is information, not just something to empty. How much is coming out, and what it looks like, are part of how the therapy is judged.
On the wound, the therapy is aiming at two specific changes. It reduces inflammatory exudate and promotes the formation of granulation tissue. So less drainage over time, and a wound bed filling in with granulation tissue, are the direction of travel. It’s the same process as ordinary wound healing: new tissue fills the wound, new skin begins to form over this tissue, and as the wound heals, the edges pull inward and the wound gets smaller.
There is a direction worth watching the other way too. Regular wound assessment should include checking for any discoloration, duskiness, or mottling of the wound bed or wound edges.
On timing, be careful. There’s no set number of days after which a wound VAC should be judged to be working or not. The closest general benchmark is the definition of a non-healing wound — one that has not started to heal in 2 weeks, or has not completely healed in 6 weeks — and that applies to wounds in general, not to this therapy. Treat it as a prompt to ask the question, not as a verdict on the device.