One time I finished a trip at LAX around 3pm, and the wife/kids were booked on the 11pm redeye so I tried to list for that jumpseat at the Spirit checkin counter at LA. They said they didn't have a way to list me for that flight, and to do it at the gate. Of course the gate for the flight won't exist until ~2 hrs prior to departure.

Is 1 hr prior the earliest I can check in at the gate with the white slip jumpseat form?