In case you miss my response,
Imagine your kidneys, heart, arms and legs were stolen and implanted in someone. They were taken from you by accident, it was supposed to be the guy in the next room. You will die unless you get your parts back. The authorities find out exactly what happened. The recipient doesn't want to give them back, because he or she would die. The doctor refuses to take your body parts off of/out of the recipient, even though a horrible accident happened.
If it were me, I'd hope some Janeway out there would help me recover from a situation where my one and only life was stolen from me, by accident, to benefit someone else. I'm not left wing enough to think the recipient has any rights to what's mine.
The moment a way to separate Tuvok and Neelix was found, they were OBLIGATED to do it. If they had not done it, they would consciously chosen to sacrifice the lives of two unwilling participants to give life to someone else. So sad for Tuvix, but his life was not his to keep. Thankfully, Janeway saw this.
This is such a stupid debate. Tuvok had the "right" to live, but not at the expense of two unwilling participants, who also had supreme "rights".
And I guess the answer to your question is: How much time has passed is irrelevant. You wouldn't get to keep someone else's money a bank gave you accidentally, why would you get to keep someone else's life?