Barnacle and Jellisac:
Harvest three: -1; harvest two more: -1
Average lifespan decrease of -1 per 2.5 harvests.
Edit: lifespan seems to regenerate in idle times.
Rocks (all of them?):
Lifespan decrease of 2-3 per fully mined rock.
First mined rock: -2; second mined rock: -3. (Avg. -2.5 per rock)
Egg Tree:
first harvest: -6 - Edit: uncertain! -4 and -3 have been witnessed (maybe TS changed how much lifespan decreases, maybe something else influences this)
second harvest*: -3; idem dito (-2)
* from people that can harvest twice.
Add your own findings!