The reality is that initial sales are usually the big sales. After the first two or three months, sales usually drop off dramatically. A book that sold 20,000 copies in the first six months, probably sells only 400-1,000 books every six months after that. Real numbers vary, of course, depending on a host of circumstances and varied royalty and sales rates. In the real world, fewer than 10 percent of all books published make it to this level.
Most books never earn royalties.
For most books, the advance is the only money you will see. My advice is to figure that your income is your advance and only the advance. Anything else is frosting on the cake. Along with the advance from each new book you sell, you can earn a very palatable income. You might manage to earn higher royalties and make a living with only six or seven books on the stands.