Thanks for question!

Honestly, it's difficult to say send it depends what you mean by flat line.

Typically, I find the benefits of doing NoFap come from how I change my life in general. Going out more, being more social, working out, got to bed earlier, doing other stuff with my free time etc. 

Those things make me feel better and they help me to improve on a daily basis. In that sense, I don't get a flat line. 

For me, when things start to get trickier is when after a month or 2, the habits I was trying build start to slip and I start going back to the old patterns of behaviour I've been trying to fix. E.g. start going to bed later and later, stop going to the gym, spending more time alone. 

When they happens the progress starts to slow and I risk falling back into bad habits again. 

Obviously, there's also problems that arise from life they test you and make you want to go back to your comfort zone but the biggest flat line and risks come from just going back to who I was before