I have no clue, but hopefully someone else will answer this.
I’d say ASAP! Dean Cornblatt at GULC encouraged the waitlisters to send a LOCI now, if in fact we still have interest. He said to not worry about hounding them. The admissions committee wants to know if they should take you seriously as an applicant. I think it’s good form to let schools know where you stand as soon as you know. And you definitely want to withdraw as a courtesy ASAP, to open up space and money for another student =)