I thought this would be a useful reminder for us from Wikipedia:
The first time it is recited is at Vespers of Clean Monday (celebrated on the evening of the Sunday of Forgiveness), the service which begins Great Lent. The prayer is used on Monday through Friday of the five weeks of Great Lent, and on Monday, Tuesday and Wednesday of Holy Week. At the end of the Presanctified Liturgy on Great Wednesday there is an additional recitation of the Prayer of St. Ephraim, at which everyone - even those who have received Holy Communion - makes prostrations. This is the final recitation of the Prayer of St. Ephraim during that Lenten season, and from that moment forward, the emphasis of the services is no longer on repentance, but on the Passion and Resurrection of Jesus.