How long is a dogs heat cycle?

Dams (female dogs) are in season (heat) for 3 weeks, but are usually receptive only during the 2nd week. Because there are individual variations, she must be kept confined from male dogs for the full 3 weeks unless you’re planning to have her bred.
Dogs generally have 2 “seasons” per year. If you want a litter, they’re usually bred on the 9th, 11th, and 13th days of their season. There are no reliable early tests for pregnancy in dogs, so it’s a wait and see situation. You may see no outward signs for the first 5 weeks.
Some dogs are very obvious about being in heat and leave small blood spots behind when they’ve been sitting. Others hide it very well and if you’re not watching closely for it, you might miss it altogether. Male dogs however WILL KNOW. So if you’re not planning to have pups, you can save your dog from risk to gender specific cancers and accidental litters while making her an easier dog to have around by spaying her at 6 months of age.