It can be anything between 6 months and 2 years, it just depends on the individual dog; I wouldn't necessarily expect it to come along at 6 months as that is at the very young end of the spectrum. Caoimhe was 11 months at her first season and Cocker bitches at work have varied between 8 months and 18 months. Some of them show some signs of being a bit hormonal, clingy, generally off colour etc. but again this varies from bitch to bitch.
I have merged the two threads on this topic as they are asking the same thing and were both posted today.