The JRT will be in heat for approximately 3 weeks, or 21 days. They can also go into heat from the 6th, 7th, 8th or 9th month. It varies. Mine went into heat on the 9th month for her first 3 heats and now we expect the 4th heat to be around the 6th month. It does change. Hope this helps, Stef, Belgium, TEXAN JRT Lover!