Sitting through a sesshin is something you need to come and experience for yourself as this is as personal an experience as it gets and no one can tell you beforehand what to expect or not to expect.
It's above all about deepening your sense of intimacy with yourself, with the breath that fills your lungs, with the thoughts and emotions that disturb and unsettle you at times, with the walls surrounding you, with those sitting in silence next to you, with the roof that shelters you, and the vast empty space stretching infinately beyond that bekons you to drop and relinquish any idea you may hold about who you are, what life is about and what steps you need to be taking in order to make yourself everlastingly happy.
I have no idea what benefits, if any, you might get from prolonged sitting in stillness and silence. The best state of mind is to come expecting nothing and the next best state of mind is to welcome seeing your expectations disintegrate.
You don't engage with this kind of practice in the spirit of gain in mind. Or if that is what is bringing you to it you soon realize if anything it is more about an experience of loss.
What this loss is about you would need to find out by yourself.
Losing some of your bagage, walking perhaps more lightly, ungrasping entrenched and limiting views about oneself and the world, sensing that there is an element of intangibility about your experience of the world, wouldn't that be a welcoming and freeing loss?