What is the life expectancy for pitbulls?

I have four pitbulls 1 male who is four years old and three females who are between 2-3years old. They have all been diagnose with heart worm. I know, this could of been prevented and I hate myself for not doing what I had to do to take care of my babies. But I applied for a credit card with a $ 3000 limit to get them treated b/c it is $ 400-600 each. I don't make alot of money and my boss made me mad by saying I could buy a car with that (b/c I am driving a 1993 pickup truck) and making me upset by asking what their life expectancy is and what it would be living with heartworms. But I was just wondering what their life expectancy is because I am getting them fixed because I want to spend many more years with them and I want them to be happy and healthy through those years.