It's a bit weather dependent as to when they start to come in to season. Onece they have started though it's a 28 day cycle which include the season itself.
my mare comes into season around march april until september, depending on weather as its the increased daylight thts triggers hormones. Normally 1 week in every month, bit like us humans!!! vaaries slighlty from horse to horse
Everytime she gets too close to another horse it seems! It kind of helps her though in the competition environment- she uses the agression to go and attack those jumps! Had her on regumate for a bit as she was very marish and I needed to do a team ride without her squirting on anyone(!), it worked but her jumping was just not the same!