
Depends on the breed! I know that many dogs would show a lot of interest around 4 weeks of the initial season no matter how much I'd bathed them, but there again in my breed a lot of them have been mated at days, 18 up to days 21. Other breeds really only show their season for around 2 weeks.
I keep my girls away from shows for a month from the first day of them showing to be in season. but there are a number of people who still take their dogs to shows. This is one thing that I wouldn't do though myself.