I'm not referring here to first lines or paragraphs, or "hooks" or anything like that. Rather, I mean the artistic "moment" when you choose to begin typing the first sentence. A key error, one that I've made, is beginning to write too early. Pouring the wine before its time, so to speak....
Let's back up. Why do we write in the first place? Is it for "honor, power, fame, riches and the love of women?" (as Grandpa Freud put it). One or more of those? Other reasons? Hey, that's up to you. But if you're a serious writer, then you must treat writing a novel with the utmost seriousness. The moment of starting a novel ought to approach the significance, the build-up of emotion and spiritual commitment of asking someone to marry you.
Okay, I said "approach." But I'm not unserious here. The big classic and canonical novels (of, say, Tolstoy, Hemingway, Stein, Alice Walker) extracted from their authors the same pound of flesh that true love takes from all of us. In other words, unless you're writing something light and funny, or within a genre (romance, mystery, western), starting a novel is to kick off a serious relationship, and the last thing you want to do is jump the gun.
Obviously, before you type the first word you need to have done the research, the thinking, the mulling over, the outlining (if that works for you). You need to have accumulated more material than you can use; writing, after all, is largely a process of leaving out the great majority of things in order to focus upon what is meaningful and "true."
I don't have the answer as to when you should actually begin writing your novel. That moment will differ for all of us. For my part, I need to feel that my characters are living, tangible people on the move through a fully-imagined landscape (a setting). I need to feel an artistic fullness akin to the sensation of being well-rested. I want a sense of hyper-acuity in terms of events and images happening around me--to be "preternaturally keen and alert", as Ambrose Bierce writes in his short story "Occurrence At Owl Creek Bridge." I want to be excited and full of anticipation.
In the end, starting a novel is like making love. If you're really serious about it, you'll take your time getting there. And you won't be sorry, later.