Each tank of Nitrox gets analyzed multiple times by me.
I analyze at the shop using the shop's analyzer whenever I pick up my tanks after a EAN fill, or after a tank previously filled with EAN is topped up with air/EAN21.
I then reanalyze at home with my own analyzer.
If the tanks have been sitting a while before I actually use them, I will double check the mix again while I am packing gear so if there is any issue, I have time to deal with it before I leave.
And finally, if I am taking them on a boat dive where my tanks may be stored with others, I check them again as I am setting up my gear for any dive.