How long does a bitch's season last

I posted a few weeks back about whether to have my bitch speyed at 7 months. I decided to wait as the general consensus was she was too young. She came into season a week after the original op was cancelled and I think she is day 13 today.

She has been spotting all this time, but only swollen and weeing lots over the last few days. As I have 2 entire boys I'm not leaving them unsupervised together, but would like to know when we will be out of the danger zone. If I remember rightly a season lasts 3 weeks? I can't remember the fertile bit though, and she hasn't been harassing them, or standing for them yet.

Any advice please?

Thanks

I would give it 4 weeks to be on the safe side. 2 of my girls don't even ovulate till days 19 and 21 respectively, you just don't know without blood testing.