
The average season lasts for 21 days, but an extra week of vigilance is never wasted. She won't be fertile the whole time, but it's impossible to predict when it will be - bitches have produced litters from matings as early as the 4th day and as late as the 20th - occasionally even later. If your bitch has read the books (;)) her fertile period will be from about day 11 to day 15.
Don't let her even go into your own garden unattended - many crossbred litters are sired by an athletic Romeo who can jump high fences!
I think you'd find a copy of 'The Book of the Bitch' by Evans and White invaluable.