Hello and welcome to Apple Support Communities, PGA_UK.
If we understand your post correctly, you switched to an Android device and deregistered your phone number for iMessages. You'd like to know how long this process takes. We’d like to help.
After you deregister your phone number iPhone users messages to you will be sent via SMS rather than iMessage. This will happen as soon as you've deregistered your phone number.
The article you were viewing has good steps to deregister and this article will show you how your messages will appear for other iMessage users moving forward:
About iMessage and SMS/MMS
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities.