That depends on the airline, the industry at any particular time, unforeseen catastrophic events, retirement age changes, and a host of other factors. I would hope 23 years would not be unrealistic, but every year that one is qualified and able to take a major airline position, but chose not to on their own accord, it decreases their chances of it happening. One rolls the dice and takes their chances, and depending on chance it could work out better either way.