As brother huge-girth stated, your erection quality (EQ) starts to drop. This is a sign of heavy or extreme fatigues. Pinging or jabbing of sharp pains. This an indicator you went to far, too hard, and too fast. This is why we always warn to slowly adjust and move up slowly with patience.
For very intense day, 1 day on, 1 day off. This is not where you suddenly get blisters and pains. This is when you push to the point of fatigues. You will be able to tell when your EQ can't go past 70%.
For moderate routines that doesn't cause you to reach fatigues, 2 days on, or even 3 days on, 1 day off.
For maintenance, you can go even 6 days on, 1 day off. This is light routines.