This is according to a friend who had it done:

If the treatment worked and there were embryos left over, they waited X amount of time, I think a year or so, and if they don’t hear anything from you then they are destroyed.

My friend said she got a reminder but didn’t want to think about it, that it was too hard, so she never responded and assumes they’re gone. She said that to her, she never told them to do it, and that helps her if she ever thinks about it. She ended up having twins.