Range at which risk of collision can be considered is very difficult to Answer.
Prior to Risk of collision we should consider the Close Quarter Situation. Which can be defined as unable to maintain the required CPA. Now the question arises CPA requirement is generally given in SMS and in Master's Standing order.
if we consider these documents we can define close quarter situation but what about the time when Master himself is at conn, at this point close quarter situation will be defined where hydrodynamics comes into play( Interaction ) and where the situation demand both vessel to take action without delay.
Just to remind all of you that above comment doesnt mean that it's ok for Master to have a close quarter situation. ROR specifically says you should never let the close quarter situation develop, Your action should always be early and substantial.
Any thing beyond close quarter will be Risk of Collision.