I change filter sock once every 2 weeks.
It is a DIY sock that I sew myself using fabric I bought from Jo-ann's fabric, so it is a completely brand new sock every time. cost me 25 cents per 6" diameter sock.
Nitrates in tank is 0. So I know first hand you do not need to change your filter sock every day or two to keep nitrates at 0.
search for DIY filter sock in reef 2 reef site.
this is the fabric
sock
with sock holder. I used the old sock holder ring from an old sock, punched holes through it and used zip tie to hold the DIY sock
this was when I was changing sock once a week, I figure I could go two weeks and make the most use of the filter sock and been on 2 week schedule for at least the last 6 months with no issue.
I don't have a sewing machine at home so I got this for like $10 at walmart. I works just fine.