Most say 6 months which is when I first felt a little kicking flutter. Usually its best when the mare is eating to put your hand in front of the udder on her belly and you should be able to feel something sooner or later.
The earliest I ever feel it is 5 months on a few mares and almost for sure by 6 months. I put my hand right on the bottom of the belly with the flat part of my hand.
I start checking at 6 months, but usually don't feel movement til 7 months. Best time is right at feeding time. I place my hand flat on lowest part of the belly and concentrate. When you feel a "no doubt about it" baby kick or roll, it will make your day