TL;DR:
| 1 star | 2 stars | 3 stars | 4 stars |
|---|---|---|---|
| ~3.9% healing | ~3.9% healing | ~3.9% healing | ~4.5% healing |
| 10 seconds | ~8.3 seconds | ~4.17 seconds | ~4.17 seconds |
| 256 seconds to fully heal | 214 seconds to fully heal | 107 seconds to fully heal | 93 seconds to fully heal |
Celeste is, as of now, the only fully premium pet in the game. She heals your hero on every 10th shots that connects with an enemy. Since you unlock Celeste at 2 stars, and in her skill tree certain perks can be bought once Celeste is 2 stars, I will assume everyone who has her will have all upgrades that are available at 2 star unlocked. This also includes the small attack speed up, which increases the shots/second from 1 to 1.2. Assuming Celeste is constantly attacking, and that none of her shots miss, (which isn’t super unrealistic, since you should have homing.) then every ~8.3 seconds you should be healed by about 3.9%. This means that if you are at 1 hp, through natural causes or through Sisu, then it should take roughly 214 seconds, or 3 minutes and 34 seconds, to fully heal yourself.
If you have Celeste at 3 stars then with double shot, then all times should be halved, to healing you every ~4.17 seconds, or fully healing you in 107 seconds, or 1 minute and 47 seconds.
If you have Celeste at 4 or 5 stars then with double shot and combat medic you should be healed by ~4.5% every 4.17 seconds. This would fully heal you in roughly 93 seconds, or 1 minute and 33 seconds.