The question of when you want to let go of the soft pedal is very dependent on how you have used the soft pedal so far... Why have you used the soft pedal so far? What did the texture added by the soft pedal mean to you? When do you want to shift that meaning to something new, to a new color, a new idea?
In other words, I am trying to tell you that the answer depends on what you have painted so far. It is very interesting because I also slightly release the soft pedal at measure 11 (for the first time). You could do that, but you have to have conviction why you are doing that.
For me, the melodic line in the left hand (Ondine's call) up until measure 10 is coming from under the water. Ondine here is still singing from her underwater world and is on her way ascending up. The soft pedal, for me, is used here to serve this texture of sound, this poetic melody that is heard but from a distance, just the echo of it ("I thought I heard A faint harmony that enchants my sleep."). In measure 11, however, I believe Ondine is for the first time reaching the surface of the shimmering and pulsating water. Notice, here, for the first time, the left hand is over the right hand (a gesture characteristic of this piece). The motion of sound has to be carried in accordance to this, in my opinion. The C# in measure 11 comes out as if she pulls her head for the first time above the water. The environment for her has immediately changed, and so a corresponding change of color and touch should be used here to describe this new environment, this new air she is breathing. Her voice (the left hand melody) should not sound any more the same ... It should project into the air more as if her singing is more free (Remember she is now no more in the water palace of her father, she is free of him. Her Father-King is someone she will clash with in the double notes measures (58-62) and he will be whipping the croaking water with a branch of a green alder tree (LH of the double notes passage))
Using a similar approach to the music, you can probably figure for yourself what you need to do in the glissando passage. I don't want to spoil your creativity by talking too much (I think I already talked to much ... and too explicitly).