I breed my female for as many years as she seems healthy and productive. This coming year (it will be my 7th season), for example, I will retire 3 females: one, I hatched in 2005. She bred every year for me since 2006 and this past year she laid only 4 eggs, all of which were infertile. The second has an abdominal hernia and produced only 1 fertile egg last season. The third is an enigma which I bought to breed, but she has increased her enigma behaviors and didn't produce any eggs last season.
same, i breed them until i can see clearly that its time for them to stop. Or if i want to sell them, like last year i sold an SHTCT 3 year old, because i got interest in it and someone was willing to pay a high price... so if i dont need her i may as well. Though that sounds harsh i normally like to keep them as pets, but at the same time i do need to get rid of ones that arent producing, cuz theyd make a lovely pet for someone not breeding.
edit: ^ god that makes me sound really mean and uncaring for my leo's, hopefully you guys get what i mean. if im not attached to them and theyre not producing eggs then thhey will be more use to someone else.