Let’s not be like Photobucket, who have emailed me every day for at least six months, possibly longer. My attitude to my old Photobucket account is “go ahead and delete it then”, and I ultimately stuck their emails in spam because I am sick of the nagging. Just! Delete! My! Account!
Honestly, I feel like a week and a single notice is enough. Even if someone belatedly gets the email, they have no account history lost. The chances someone has signed up in the meantime with their desired username is small (and is it still their desired username anyway, after this much time?). They can just sign up again. We’re not blocking their way back.
(It may be worth saying that in the email too.)