welcome to the board, jenny.
this is so different for each person.
for me, i felt the "i think i wanna leave" creeping in about four or five years before i actually uttered the words.
i do remember clearly when i knew - it took about two years after that.
and i do remember clearly when i really knew, meaning i was going to actually say the words out loud. to him.
once i made the commitment, i followed through right away.
never looked back.