More or less every session. I often say "please freeze time so I can stay!"
Or if I'm in a more trusting mood I'll just refuse to let go when it's time to hug goodbye. I'll tell her I'm not going, too bad, I'm not letting go. Obviously I DO let go, or else she tickles me and MAKES me let go. Playing sometimes makes the goodbye easier.
The nicest thing she says is she often says "I don't want you to go either. I love seeing you. But that's how it works Bay. We only have so much time. In a different world where things were different I'd let you stay as long as you wanted"
THAT made it easier.
But now we really start the ending part about 10 minutes before its time to leave. We have several rituals we do to ease me back into the world. That helps too.