How long can the consecrated host remain in a pyx or lunette before it must be replaced?
Canon 939 states: “Consecrated hosts, in a quantity sufficient for the needs of the faithful, are to be kept in a pyx or ciborium and are to be renewed frequently, the older hosts having been duly consumed.”
Concerning the meaning of “renewed frequently,” The Canon Law: Letter and Spiritcommentary says, “The hosts are to be renewed ‘frequently,’ in order to avoid danger of corruption” (512). To prevent corruption of the sacred species, canon 934 §2 requires that “[W]here the blessed Eucharist is reserved there must always be someone who is responsible for it, and that as far as possible a priest be available to celebrate Mass at least twice a month.”
According to Redemptionis Sacramentum
It is highly recommended that at least in the cities and the larger towns, the diocesan bishop should designate a church building for perpetual adoration; in it, however, Holy Mass should be celebrated frequently, even daily if possible, while the Exposition should rigorously be interrupted while Mass is being celebrated. It is fitting that the host to be exposed for adoration should be consecrated in the Mass immediately preceding the time of adoration, and that it should be placed in the monstrance upon the altar after Communion. (RS 140, emphasis added)