Not long at all; in fact its usually instant. Once you’ve made the payment for it, you should be sent your policy documents immediately via email.
Impounded car insurance is usually third party only, so your car will be legal to drive but will only have minimal cover so you can get it released. This is why it’s a good idea to seek regular car insurance as soon as you can once you have your car back.
This is will vary depending on things like the make and model of your car, why it was impounded and your age. It will also vary depending on the insurance provider you get cover from.

