I guess at the start of last before last-year was when I'd reliably say outright that I could consider suicide. So, I can say I was only "serious" about it for... God, almost 2 years now.
Though on the other hand, I did distinctly feel a gentle "wish to die" somewhat in middle-school. I just simply didn't see anything in my life at all. At that time it was quite minor and I could ignore it with the help of all kinds of distractions. Although once when I was feeling depressed, I mentioned to someone I wanted to jump of a bridge and die, they just laughed it up /w me, the funni autist, saying it (holy shit that could've been a major slip-up).
While I've always felt there wasn't much to life, since early childhood. I simply couldn't care or thought of it under a vague optimistic light. But now, the nature and inevitability of what my life will be is setting in.
"seriously" for 2 years...
"Subtly" for about 4-5.3 years.