I'm really sorry about your cat.

The advice to get a second opinion is a good one.  And if possible, talk with a
vet (your regular one or the second opinion one, whichever you feel most
comfortable with) about prognosis and quality of life.

When my first two cats were terminally ill, I had a long talk with my vet after
each diagnosis.  We discussed the likely progression of the disease in terms of
symptoms, quality of life, and probable life expectancy.  This was not all
concrete information, but it made me feel a little less powerless to have it.

Early on, I assumed that if my cats were having two good days out of three, I
wouldn't think about euthanasia yet.  And that's how it worked out for a while.
In the end, one went abruptly, and the other had a steep decline about 4 days
before I took her in for the final vet visit.  To go back to the question you
asked as title to the thread, in the first instance, it wasn't my decision, and
in the second case there was no doubt.

This is something you have to feel your way through, according to your values,
your relationship with your cat, and what you know of your cat.  The important
thing is to know that you are trying your best to do what's right in a
difficult situation.  So try not to second guess yourself or let other people
do it to you.  There's no absolute answer here.

It's tough, and I feel for you.
Daria
Timing is everything.