How soon how far and how fast I open gate paddles varies from lock to lock - on a shallow lock they might be underwater to start with, so they can be opened early and quickly, on a deep lock with no baffles, I would start once the water is up to the bottom of the gate paddle and continue to gradually open as the level rises, always being prepared to drop them if necessary - I think this bit is where beginers can get in trouble by just standing there watching as the boat sinks rather than dropping the paddles and sorting it out.
Therefore I would say that if you know what you're doing there is no hard and fast rule, but if you are unsure then never open them until they are fully covered.
Of course, some locks only have gate paddles so all you can do is open them carefully. I can't recall where I've seen this, and by now this might have changed, but I'm sure I've seen it somewhere?????