I have a friend who's queen calls more than she'd like. She will come into heat when her kittens are 8 weeks old. When she's in heat she will stop eating among other things. My friend is forced to breed her more than she'd like because of a) the risk of pyometra & b) the fact that she loses condition by continually calling. Even her vet has told her it's better to mate her. That's not to say she's constantly pregnant or nursing. Friend does hold off, and let her have a couple of heats, and also, she comes off call during the winter months, so friend & cat get a break from the calling.
I think as a rule here (don't quote me), you're not meant to breed more than 3 litters in 2 years. But, as everybody else has said, it really depends on the individual cat. It's also not good if you have a cat who's coming into heat every couple of weeks.