Avatars are updated once a week. This is largely to prevent constant and frivolous avatar changes. With a once-weekly update, the average wait is 3.5 days.
If it's been 8 days, and your avatar still hasn't been updated, please re-send it to firstname.lastname@example.org
. Chances are, it got stuck in a spam trap or was otherwise not delivered. Other common problems include not sending the pic to the correct email address, not including your username, or neglecting to attach the pic to your email.