The Ninety-Ninety Rule is attributed to Tom Cargill of Bell Labs and states the following: "The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time."
The quote was made in relation to computer programming but one of the underlying themes of Tools not Rules is that the concepts translate across disciplines. These ideas work for illustrators, designers, bakers, musicians, dancers and more. The Ninety rule is no exception.
Don't underestimate the amount of work it will take to bring a project to a close. That first big push, coming up with the ideas and laying the groundwork is the easy part. Wrapping up everything (bug fixing, prepping marketing material, getting approval, adding audio, clarifying the UX, etc) so your project is ready for world is the hardest part. If you're an experienced project manager you might have accounted for all of this but if you're just getting started, or working in a new domain, it can't hurt to plan for the final 10% of a project to half, or more, of your production time.