After the crops are grown, how long are they available for harvest? Is the watering and harvest calculated the same as the construction of buildings? For instance, will the watering time be shorter if more villagers are engaged in it? Will the harvest produce more food if more villagers work at it or will it just produce the same quantity of food but be harvested faster?

once the crops are grown, they stay there until all of them are harvested. they won't spoil or die.

watering crops doesn't affect how long it takes for them to grow. it's just something that the villagers like to do. if i see mine watering crops, i make them do something else since watering is really pointless.

the crops always produce the same amount food. if you put more villagers on farming, it'll just take less time for all of the crops to be harvested.