I get the nuances and realities of what everyone is saying. But to be frank we may put one, two, three, or whatever years writing a book in the hope that it will get published and that we will get a large advance (I realize that some writers prefer a smaller advance and potentially quicker royalties). Me, I want as large an advance as I can get. I'll worry about later, later.
CheshireCat makes a valid point. To a large degree the advance depends on many factors, including sub rights sales and the print run. Case in point is my first published book (I was still in college) about 30 years ago by a major publisher. The advance was very large! It was very large then and in today's money would be well over $200,000. At the time I didn't have an agent, just me and an intellectual rights attorney. All went smoothly. However, what we didn't know is that the publisher already had 35 different foreign sub rights publishers lined up just waiting, and 3 major US book clubs. They didn't have to disclose this info in the contract since prior to my signing same they hadn't, and weren't able to, sign off on any of these deals (they not only didn't have the rights I sold them, but no idea when they could deliver books--or the manuscript--to these companies, and in their defense they didn't have any executed contracts). So, even though I received royalties of 5 to 10% on the retail sale price on these editions, was the advance as large as it should have been? Should there have been some caveat added to my contract? All speculation and frankly unimportant. Of course I'm not complaining, but I am trying to make the point that all that glitters is not always as golden as it might appear. Of course if I had an agent at the time (which I didn't think I needed--foolish me) I'm sure s/he would have made a better deal.