Absorption effect "cooldown" is a bit inappropriate, as it refreshes absorbtion hearts every 30 seconds.
1. Get a golden apple (with ingots)
2. Eat it and get absorbtion effect
3. Take some damage
Now you have to wait 25-30 seconds for those hearts to refresh. If you wait for 25 seconds before taking damage, absorbtion hearts refresh in about 5 seconds, allowing you to take more damage in total just because you took damage at the right time. This could be exploted in pvp intentionally or just make it frustating because enemy was lucky to fight at the right time. I would suggest refreshing it after 15 seconds of not taking damage or something similar.