Dogs will experience their first heat cycle between 6-12 months of age, though most come into heat between 6-8 months of age. It is important to spay your dog at 6 months of age to prevent heat cycles. Spaying a dog before the first heat cycle reduces the chance of mammary cancer to almost zero. This also prevents unwanted litters--it is amazing what a male dog will do to get to a female in heat.
LOL, it IS amazing and doxy's end up c-section fairly often as well as the dangers of pregnancy too young (among other things the pups taking the calcium the mom needs can result in death within 24 hours of onset if eclampsia occurs which is frequent in a dam too young as SHE needs the calcium for her growth and too much calcium is also a problem). You REALLY want to avoid it. Doxy's in particular are usually 5 to 7 months (with the average of six) though 4 isn't unheard of.
After a liter how long before they go back in heat