It took me about 6 years to end up with what I feel is a solid story for my comic, and I'd rewritten it several times since I started thinking seriously about it. I think what really helped in my case was finally going "okay I've been fiddling around with various character ideas and the main premise long enough, I need an outline."
Once I figured out how I wanted my story to end and what the main conflict would be, the rest just kind of fell into place by working backwards from there, if that makes sense? It really helped when it came to working out character arcs as well. I did have to toss aside some cool scene ideas and stuff that I'd clung to since the beginning since they no longer made sense, but really it was for the best.
So yeah tl;dr - Once you have your main plot and ending figured out, as well as the character arcs for your primary characters, I'd say that's a pretty good launching pad to start from.
You don't need to have it all scripted out to the most minute detail before you get started on pages either, I've been keeping the details like dialogue and paneling fairly loose just so stuff remains interesting to me as I work on it. So long as I know what needs to happen next, it's all good!