How long do chickens lay 'pullet eggs'?

I've never had BO, but large bodied fowl take a bit. The hybrids that are bred for commercial purposes, primarily, sort of get on with large egg laying quickly.
The more traditional birds can take up to a year to hit their full stride. I've never, ever, been overly concerned about egg size. In fact, I wince at the large size eggs that young hybrid pullets lay. Personally? The size will come naturally, over time.