Answer: I thought I knew the answer to this question – and then I went to look it up... The short answer is, “whenever it is appropriate.” So, when is it appropriate?
Fr. Stanley Harakas says the following (excerpted):
We mark ourselves in (the sign of the cross) to accomplish a number of different things...; to indicate our acceptance and concurrence with words, prayers or actions (e.g. in worship - when a blessing is pronounced or when the Holy Trinity is mentioned)...; as an inaudible prayer when we wish to invoke God’s presence or express thanks...; as a blessing upon ourselves when, say, we begin or end a journey, when we begin or end our day...; etc.
Marilyn Rouvelas, in her book, A Guide to Greek Traditions and Customs in America Second Edition, gives a list of 14 instances when the sign of the cross should be made:
You should also make the sign of the cross every time you pass an Orthodox Church... Why? Because the Body and Blood of Christ are resident in every Orthodox Church in the presence of the reserve Holy Communion held within the “Ark” on top of the Holy Altar. Therefore, every Orthodox Church is sanctified by the actual presence of Christ, and we honor His presence by making the sign of the cross.
Occasionally, a parishioner may drift away mentally from the rhythm of the services and then use the sign of the cross to ask God’s forgiveness or to get themselves back on track. So you may see parishioners making the sign of the cross at times you wouldn't necessarily think was "the time to do that". In one sense, there is not a time that it is "wrong" to make the sign of the cross. We do have times that it may not be necessary as you are already receiving a blessing. However, if it aids the participant to make the sign of the cross as a means of personally accepting that blessing, I won't tell them it is incorrect. So, make the sign of the cross whenever it is appropriate...