How long after breeding is a bitches next heat cycle, on average, for I know that no two girls are alike. Is it 6 months from the time she had her last litter or 6 months since she was bred?

I would say 6 months since she was bred.

Theory: 6 month after last heat.
Practice: Who knows
My experience is that when you breed them, they take a little longer to come in season again.
Some females come in season once a year or every 10 months. The is no golden rule.

I find that 7.5 months happens more often than 6 months.

At least as long as they generally go between seasons. Probably a little longer.

Mine go a little longer after a litter. So if their 'regular' cycle is 6 months then they come in at 7-8 months. If regular is 8 months, then it's 9-10 months.

Mine also are more like 7 to 8 mos. after last heat.

There is no golden rule at all, but if your girl is like clockwork you can usually come close by determining when she was in season last time. Shouldn't matter if she was bred or not.

Had one girl come in every 9-11 months from 2 years to age 6. She had a litter at age 5 and her next season was when the pups were just 7 months old, so she was on the low side of her range.

Another bitch had seasons that were closer. (usually only 5 months apart) She came into season again when her pups were only 12 weeks old.

I've also had girls that have come in a little late.

Basically, there is no magic formula or crystal ball. Sorry, I sure wish there was, though usually it doesn't matter for me. I've only bred one bitch back to back and it was an incredibly fit girl who cycled once every 11 months or so. She looked better after 3 litters than many girls I've seen who've had only one.

My girls follow the same cycle, 8-9 months, whether they are bred, or not.