To add a data point... I am an Elite and used the free same day confirmed benefit for the first time yesterday. I tried calling ~40 hours before my scheduled departure to see what the rules are and was just told they couldn't do anything over the phone, go to the airport. So I went with some friends who were already heading to the airport and requested to change to the flight a full 24 hours before my scheduled one. The ticket counter agent said there was only one seat left on the plane and I could have it!

I asked him if there were rules about same day confirmed and standby published anywhere, and he said Frontier doesn't even tell them what the rules are. So I imagine this might lead to frustration in some cases, but I got a great result this time. This was at LAS which is a focus city, so not like it's a random outstation with uninformed contractors.