ANytime ealry season can be good. Late season as birds are more wary, I use them less. Keep in mind, the hen is supposed to go to the tom in nature so sometimes, a tom will hang up if he sees a hen decoy expecting she'll go to him. Also keep in mind that a tom might get nervous if he sees a decoy not moving and calling coming from a different spot as he gets closer.
All in all, decoys have worked very well for me. I use hens, jakes and toms at times.