Ten days is a nice number, but if I'm in a hurry I remove them after a week.
The longer the stronger but after a week you get diminishing returns.
I have tried testing the strength using sample bricks and breaking them. I could find no measuarable difference between curing under water a week or a full month.

cd2, If it's a planter/trough I leave it for 3 days in the water bath. I change the water every day. I have a friend that makes planters all Winter in his greenhouse and when he unmolds, he places them under the benches in his greenhouse. When he waters the plants he waters the planters. After a couple of weeks his planters are leached and he adds new planters. What are you making? Billie