Usually it is going to last about 3 to 4 weeks.

The first week is non-bleeding, you'll notice your male dog start sniffing and licking the female's urine.

During the second week, sporadic bleeding occurs, the vajajjay begin to swell. Your male dog will start to go crazy, if you don't put a diaper on the female, the male will follow her everywhere and lick off her blood, if blood gets on the carpet, immediate pour hydrogen peroxide over it and it will clean it right up. If you put a diaper on the female, the blood saturation is greater and your male dog might get a little more crazy. he will start to urine mark everywhere (leaving business card for the female to consider him) by the end of the week, your male dog should be smart enough to figure out how to by pass the diaper, so you must supervise them, when in doubt, back to the crate and let the howling begin :)

During the 3rd week you may get a little sleep if you're lucky, there are times that I have seriously consider to let someone else baby sit one of my dogs, your male dog have officially lost it by now, howling, nose nudging, he will totally drive your female NUTS. your female dog may finally give in due to exhaustion, by now your male dog might not have eaten for a few days and try to get it on for the last couple of days.

Week 4 should be back to normal, your male will start to sniff less and hump less, you'll be a much happier person because you finally sleep through the night :) Congrats! you've made it, see you again in 6 months!!!

My female just went through her first heat cycle a month ago. I have two male corgis also and they did not have any reaction to her. They are both fixed but the vet said it was still strange that they did not pay any attention to her.

She did so well except she got a bit moody for a couple days in the 1st week she was bleeding. The vet told us to try the dog diapers to keep the blood off the furniture...it worked. She did not mind the diapers at all. If you try them though be careful of the tail hole that they put in the diaper for normal dogs. If you have a Pembroke the hole in the diaper slides down and she can bleed through it! After a few days the diaper became a routine and she would come in from outside and stand there and wait for us to put her diaper back on. She did bleed a lot during the 2nd and 3rd week! We thought it would never stop. I recommend a thin maxi pad inside the diaper if you go that route!

Each season can be a bit different. Some dogs clean themselves far better than others. Typically the cycle runs between 21 - 28 days. To be safe I would keep them apart for the 28 day period. Understand too that two dogs can mate very quickly. I know some very knowledgeable people that have experienced this. Supervision and a secure environment is most important.
Heat cycles vary in dogs. I know some that cycle every three months and some that cycle every nine months. Some dogs cycle and there is very little physical evidence. Typically it is recommended that the dogs be spayed/neutered. Living with intact dogs takes far more managing then does spayed/neutered ones. Behaviors are usually increased. Do know that diapers do not prevent breeding.