Porting phone number out of U-Verse, question on timing

I'm assisting a family member in switching from U-verse to the local cable co, because U-verse has been unreliable, expensive and slow, this includes porting the phone number to the cable co. Just wondering how long a port traditionally takes, also, when should I call AT&T to cancel service? Should I wait until the number is fully ported over and working on the new provider, or can I call and cancel right away? Thank you