I usually eat a piece of prosphora every day (Mon - Sat), then get more on Sun. Personally, I find that if I have any longer than this, they mold. I take this is a sign from God reminding me to go to Divine Liturgy!
Some people also eat it all on Sunday. I haven’t personally heard of anyone keeping it for an extending time. Prosphora is meant to be consumed as a blessing. Keeping it permanently and not eating it seems to defeat the purpose of prosphora and deprive one of the privilege of that blessing. Just my opinion, which is hardly infallible.
Each church should have some holy water stored. Water is blessed on Theophany (Epiphany), and then more is blessed throughout the year as need arises. I don’t know about sanitation in regards to holy water. It is holy, and a means of partaking of Grace, I think health is of secondary concern. If Easterners cared about health that much, we wouldn’t take communion from a spoon. I haven’t heard any complaints about sickness. If it works, why change it, I suppose.