It can be dangerous to just look at the indicators - the other driver might have forgotten to turn the indicator off or is indicating too early. It’s the vehicles you are not aware of that can present the greatest danger. Use your indicators even when you think you are alone. For example, signalling before a junction to indicate a turn just after the junction can make another driver falsely assume that you will make a turn in the junction - they can pull out in front of you and cause a crash. While signalling early is preferred in most cases, it can be dangerous if there is more than one opportunity to turn. Once a turning manoeuvre is completed, turn off your indicator. You must always signal for long enough to give sufficient warning to other road users.
There is no set amount of time before a turn that you should signal - each situation is different