When I can write it with confidence that the parts that make up the story are accurate - setting, hobbies, interests, etc. As noted by another writer yesterday in the comments, it's critical not too have every last detail in there about what you learned, but you do need enough to give an authentic flavor to the story. I liked her cooking analogy.

I guess... when I feel I know the world well enough to fill in all the little details that I haven't planned out yet.

To use another cooking analogy (wink, wink lisakothari), it's sort of the difference between baking a cake the family's gonna smile at and devour vs. the one lots of wedding guests are gonna fawn over and the couple's gonna freeze. You'll know when the cake is baked, you'll see when it's frosted, and you'll know in a single bite (one page!) if it's any good. If you're doinking around with some fancy contraption to create frosting flowers or to scrawl messages in various frosting colors, and sprinkling edible glitter or whatever, then you've ventured way past "necessary" and "done". You don't need to be a professional cake maker/decorator!! You do, however, need to be able to bake a decent cake. Chocolate frosting is a bonus!! But hey, if ya wanna stick with the simplest brownies, I'm sure your family of readers won't mind. As long as they taste great! Some might argue "but I like to make frosting flowers, even if it's just on an angel food cake!!" That's fine -- just don't let it keep you from getting the cake to the table in time for dessert! For my current project, I have three "facets" I started out knowing very little about: Mixed Martial Arts, Architecture and Opera. I could eat myself sick in research if I allowed it. Trick is to GET AND KEEP WRITING, and use what I've garnered like a simple chocolate frosting. For first draft especially, I can't bog down in thinking I don't know everything I need to know. If I have the right amount of salt in my cake, it'll be fine! Trouble is I love frosting!!! I've had such a great time reading books on MMA fighting, bios and what-not -- it's fascinating and fun. I just have to realize I've veered from mere research to pleasure reading now and that it's no longer "necessary" and I should only hit those pages after WRITTEN PAGES are done for the day!!