Re: Janeway's Decision to Kill Tuvix
I don't remember if it was specifically stated in the episode, but I got the impression that the longer Tuvok and Neelix were existing as Tuvix, the odds became less likely they could be recovered. Sort of like the window of opportunity to reverse the procedure was closing.
Dr. Howard, Dr. Fine, Dr. Howard: For duty and humanity! --Men in Black (1934)