I guess it depends on your browser’s cookies.

If the edit button is not there, the letter stays here forever. If you want to delete them, you have to copy and paste all the links to them to the feedback section and ask the mods to delete those letters for you.

byebye op but whyyyyy just send the letters to the museum of broken hearts

I think it has to be edited/deleted from the device you sent it from. for my letters, some I sent on my iPod Touch, and I can edit those ones fine on my iPod no problem, but the rest of them that I sent either from my phone or my laptop only display the option to bookmark them (despite the fact that theyre already in my bookmarks). if you cant delete or edit letters because you cant access/remember the device they were sent from, then send a feedback letter with all of the URLs of the letters and the mods can hide or otherwise erase them.