The only thing I've tested in over a year is salinity, always during water changes. That's about it. Probably not ideal, but it works for me.

I have a doser coming my way, so I'll be testing everyday to get my
Dosing amounts in check, after that I'll test monthly if that.

Testing went the way of the dodo bird a couple years back for me.
25% water changes every week with D&D salt my nano is looks great and with no loss of life.