How often to vac the sand with water changes?

[Stanley-Reefer]

I was previously doing the sand with EVERY water change, but since starting Prodibio I made myself a promise to leave it alone--and it has stayed bright white in both tanks!

My question is how often do you all vacuum the sand? I only do the first inch/inch and a half anyway.

 

[xtm]

I don't vacuum the entire sandbed - just the top layer where most detritus are, and I do this every water change. The area where I vacuum the most is the sump bottom.