Fed daily, for as long as needed. My mare is only marish before her 1st Spring season (even when she has been coming into season all winter), so I start feeding it when she starts playing up in March (always mean to start before she plays up, but forget!!). I have found with her that one tub does the trick.
its good stuff but takes a while to get into the system. i used to give it all in the morning through spring / summer. it all depends on your own horse though!
hope this helps
the feeding instructions are on the box. you should probs feed it throughout the year so that it stays in her system and will be more effective at the time of year she becomes marish. there are no side effects feeding it throughout the year and was every effective at calming my mare down.