I'm trying to progress through akane's pleasure realm but I can't get past the one shadow I've searched and I've seen people with a higher time on their shadow step but nothing explains how to increase it's duration 

I don't know where you've seen people with larger counters, but there is no “legal” way to change the stealth time.  If you want to increase the stealth time, you need to edit the game files a bit.

 
Open the game folder > www > data > Map111.json (open with notepad) > find “StealthTime” > edit the number after this word, the time is in milliseconds, the default is 240ms or 4s > save > run the game.