How do I optimize my ad credits for maximum potential?

I don’t like the new sponsor system nor am I familiar with how it works. I am currently in the release stage of my game which has gone pretty rough so far because I haven’t been able to reach an audience. I’ve tried spending ad credits on my game but it only spends a tenth of them. I currently have 68.04 credits remaining. How much should I bid and over how much time? My game is a horror game directed to users 13+.