My oldest female is 16 years old. She's had babies, every other year, for the past 10 years. So, she's had 5 litters.
I plan on breeding her again this year. This will be her 6th and final litter, because I feel it time for her to retire. She'll be 17 when I "retire" her from breeding. She's given me some nice babies over the years, but now it's time for her to be "just a pet". 
I know I could probably breed her for longer. I've heard of people breeding boas that are 20 and 30 years old. I've also heard of boas dying from pregnancy complications. I've been lucky with my girl. She's never had a problem. I want to let her retire, before she's "burned out". Having babies takes a lot out of them. As they get older, it takes longer for them to "bounce back".
The first time I bred my girl, she took about 6 months to get back to her pre-birth weight. Now it takes her almost a year. I don't feed my female boas heavily to put the weight back on. I feed them their usual meals.
The rest of my female boas are still fairly young. They range in age from 5 to 11 years. So, boas can breed for a very long time. If you're lucky, you won't run into the complications that come with breeding boas.