According to TibiaFandom:
item | amount | percentage | tries to get 1 |
You manage to mine enough chunks of coral to fill one crate | 121 | 9,7% | 11 |
It seems you disturbed a Deepling worker. | 776 | 62,18% | 2 |
The coral you mined was crumbly and of minor quality. | 351 | 28,13% | 4 |
soure: https://tibia.fandom.com/wiki/Liquid_Black_Quest
Unfortunately the only source with this kind of information you can find on Wiki, which is created by players and we can not consider it as official info, however amount of tries seems to be high enough to identify helpful statistics.
From my own experience I can say that gatherigh 10 Crates Full of Coral takes about 30-45 minutes.