[tirant]

In general, the requirement is the following:

a) Check for the possibility of the maneuver; b) signal the maneuver; c) perform the maneuver.

However the signaling needs to be done in a way that it helps other road users to read and act according to your maneuver, so 3 seconds seems to be a good amount of time for that.

There are, on the other hand, situations where signaling the maneuver is also desirable even though the maneuver might not be possible yet: merging into a full lane, so vehicles might free up some space to let you merge.