Depends where you cancelled it.
Assuming it’s a straightforward order with no options added or paid for,
If cancelled at the dealers it’ll be when they actually go onto the Motability system and action it.
If over the phone with Motability they’ll usually tell you there and then during the phone call.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!