About the only way to always have eggs is have a few spring hatched pullets every year. First year pullets don't normally moult their first fall and if they start laying well in early September, tend to lay right on through the first winter without a hiccup. Of course, the next fall, they too will be older birds and moult.
Most hens will lay reasonably well for two years and some then, go into a severe decline while others decline almost imperceptibly. This depends on the breed and whether it is a high production bird in the first place. Just in my experience, a typical hen has about 1000-1200 eggs to lay. The question is how fast will she lay them? The hybrids are designed to lay them quite quickly, over the first two years.
Some folks "turn their flocks over" every two or three years, cycling in young pullets every year, while cycling out older birds. These are personal management decisions.