Reviving an old post, was thinking of how I do this and wanted to add my 2 cents.
I tend to rez change at 12/12 transition like many here. Generally just to have a clean slate and bump my EC. I dont change nute formulation- Jack’s 321 straight through.
After that’s it depends how busy I am.
I target an EC of ~1.4 in flower.
If everything is going well then I can just top up.
Going well is, water level is dropping & ph is stable or rising slightly while ec is stable.
If my EC rises significantly, say to 1.6 or 1.7 I know the plant is taking up water faster than nutrients, and likely I am getting a build up of something.
I’ll generally top back up with water to bring it back in line. If I notice it again, it’s a rez change.
Also generally fall back to, when in doubt dump it out. If things look off in the grow its change out time.