Here's a portion of Dixie's reply to a similar question on another thread...

A heat cycle is 21 days long. 1 week coming in, one week in, and one week going out. Anywhere after day 7 to about 16 are real prime times for a bitch in season and I use this word purposely because when they are in they will let anything and everything who tries to mate have a good time!

They become different in attitude and will go out of their way to find someone who will give them what they want so never let her outside unsupervised, even for a second. Male dogs can smell a bitch in season for up to two miles away and will travel! Even if they are usually really good at staying in the yard!

You can put doggie diapers on her if she spots alot. Or you can do it the homemade way. I use a pair of my husbands underwear since they have the opening that works for her tail perfectly. I line it with self sticking maxi pads