Quicksand Fans

Karen and Anita are walking along the sandy river bank. The weather is warm and the idea of taking a skinny dip in the water is appealing. They come across a bend in the river, where the sand juts out across the water, providing a scenic view. Karen has her camera out and takes a few snaps. Anita gets the idea to take a topless photo "for the boys". She whips off her top and walks to the middle of the sandbar, posing with her hands over her breasts in a teasing way. Karen takes several photos, but suddenly stops. Anita looks up at her...and she realises that she is looking up at her! She looks down to see her calves sinking into the soft sediment.

"Don't move," Karen says calmly. "I think it might be quicksand." She tests the sand, but her feet sink as well. She has to go and find something to reach Anita. "Don't struggle, okay? You'll sink faster if you do."

It's easier said than done. Anita watches apprehensively as the sand slowly but steadily creeps up her legs. The warm gritty sensation is unnervingly pleasant. Minutes pass, and the sand is around her thighs. The moisture soaks into her shorts and finds its way into her private areas. Past her hips, the quivering sand continues to claim her body. She holds her arms for balance, and soon her breasts come into contact with the shimmering surface. Karen is still nowhere to be seen.

At what point does she begin to panic?

I would think Anita is going to begin to panic when she finds herself knee deep. Depending on the thickness if what she's sinking in, she's likely already having trouble moving her legs if she starts to struggle which is what most people naturally do considering the initially reflex is to try to pull yourself out.

I think that most normal people especially with some outdoor or touristic experience (with crossing some rivers or swampy forest areas) will not really panic before mud will be at least waist deep (especially if sinking is not too fast). Quicksand is more strange thing for me, but may be more familar for people who live on sea beach e.t.c. I think that panic (not hysteric) in this case may start between waist and breast deep. Some cryes "where are you? I'm still sinking!" e.t.c. with attempts to slowly get a little outside of mud. With growing (to uncontrolled, forced struggling) panic when mud getting to armpits-shoulders level.
But for me, it's more sexual when girl is relatively calm until her finish and continue rationally trying to escape till disappear and after it too (with several emerges for air e.t.c.), understanding that she really dying and totally can't escape in very-very end.

That depends on a number of things. This includes the psychology of the person and the situation they are caught in.

How remote is the person? Are they alone? How close is rescue? What is their level of survival skills?