This question is like asking how much weight you'll lose if you diet for 30 days.
As a very rough guide though:
Addiction level: Not addicted, just doing nofap as a challenge or for moral/religious reasons:
How you'll feel after 30 days: Exactly the same. I really hate these dumb articles you see now and then with a headline like "So I tried nofap for 30 days and this is what changed (nothing)". Yeah, no shit nothing changed if you didn't have a problem to begin with. If you have a real problem with PMO then nofap isn't just something you "try" and no one racks up a 30 day streak right off the bat if they're genuinely addicted.
Addiction level: Mild
How you'll feel after 30 days: You'll probably be feeling better - more energy, more confidence, hornier.
Addiction level: Moderate
How you'll feel after 30 days: In all likelihood you'll have hit the flatline by this point so unfortunately you'll feel worse than before you started but greater rewards are on their way, you just have to stick it out.
Addiction level: Severe
How you'll feel after 30 days: Just as shitty as you did 30 days ago. If you're a severe case then you don't even really start on day 1, you start from negative and need to go on a streak just to get to zero. On the plus side, when you do come out of the other side you are going to feel the biggest difference compared to those who haven't been damaged quite as badly.