When is it canonically proper for a person to make his First Confession prior to his reception into the Church?
The Rite of Christian Initiation of Adults does not give a specific time frame for going to confession prior to being received into the Church:
If the profession of faith and reception take place within Mass, the candidate, according to his or her own conscience, should make a confession of sins beforehand, first informing the confessor that he or she is about to be received into full communion. Any confessor who is lawfully approved may hear the candidate’s confession.
The candidate should probably consult with a trusted priest or spiritual director as to when he or she is ready to make a first confession. But it can take place anytime prior to being received into the Church. Most RCIA programs recommend that the candidates make their first confession within a week of being received into the Church.