From Brotato Wiki
Iframes
Iframes, or Invulnerability Frames, are a duration of time after taking damage, where you cannot take damage again. This is what prevents you being hit more than once when you walk into an enemy.
The Iframe Duration has a minimum of 0.2s and a maximum of 0.4s.
Iframe Duration = 0.4s * (percentage of Max HP taken as Damage)/15%
This means that you get the following Iframes depending on the amount of damage you take:
| Dmg Taken % | 7.5% or less | 8% | 9% | 10% | 11% | 12% | 13% | 14% | 15% or more |
|---|---|---|---|---|---|---|---|---|---|
| Iframe Duration | 0.20s | 0.21s | 0.24s | 0.27s | 0.29s | 0.32s | 0.35s | 0.37s | 0.40s |
In Endless Mode Iframes are reduced by the Endless Factor.
Self-Damage
Self-Damage done by 
Blood Donation, 
Bloody Hand, or 
Sick comepletely bypasses Iframes. Taking Self-Damage doesn't trigger Iframes, and you can take Self-Damage while the Iframes are active.