Here in Belgium, if it's by physical mail, the delay is legally defined.
e.g. Resigning by signed letter is assumed to arrive 3 business days after you send it off. Regular mail requires 3 attempts (e.g. bill + 2 reminders) before it's reasonable assumption it arrived at all.
But then again, if you wonna change highway lanes with a broken blinker, it's perfectly legal to stick your arm out the window, so maybe some laws just aren't up to date here.
Wow, this site has more donation ads than Wikipedia!