The terms are actually quite clear in the contract they sign - less than 24hr notice, they lose the session. No ifs or buts or conditions. Less than 24hrs, session's gone.
However, since if they lose the session we PTs get paid but piss off the client, and if they keep the session we don't get paid but make the client happy, in practice it's up to the individual trainer.
Basically, to me the thing is that the person should let me know they'll miss the session as soon as they know. So if they wake up feeling sick and give me 15 minutes' notice, well, they told me as soon as they knew. For example once a client's car battery went flat on the way to the gym, he called me while waiting for the RACV - fair enough.
But if a person goes out and gets pissed the night before, or if they stay up till 0300 and sleep in and miss it, or if they were sick on Thursday, Friday and Saturday, but didn't tell me about the 1400 Sunday session until 1345 Sunday, that's not good enough, I charge them.
As well, everyone fucks up from time to time, so if a person is usually reliable but missed this time, well... But if a person is frequently asking to change session times the night before, and often late, and often hungover, etc, then... I'm not going to cut them much slack.