There are two types.
There are freebies where she's making up for something -- a cancellation or two or three, or a bad day, or having had to rush a session for some reason, or .. whatever she feels she's done that's worth an hour of uncompensated time.
Then there are true freebies. Maybe it's because she likes you, or wants to thank you for something you did for her, or she wants to do something nice for your birthday or Christmas.
It happens. Very rarely, if ever, for some guys; more frequently for others, depending on how frequent a client they are and how well they're liked.
As has been previously noted, never expect one, ask for it, or even hint about it. Don't even tell her what you really want for Christmas.
It's hers to offer and give, and, if it ever happens, the fact that the idea was hers alone can bring as much joy as the act itself.
Just my 1/50th of $1, and worth what you paid for it.