When to replace sand in sand filter?

Hello, fellow pool people!

Was going to read advice here in the forums on when sand needs to be replaced and so am registering and posting this FAQ, no doubt. Thanks.

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My pool is about 12 years old. 36' x 16', rounded corners
In-Ground, Vinyl, Chlorine, A/C heat exchanger, Central IL

Recent addition was the FPH3 Pool Heater from HotSpot Energy.
With this, my house air conditioning dumps the heat into the pool instead of the air.

Motor is USQ1152 (replaced it three times now?), 220V
Pump is Sta-Rite P4R6E-187L MAX-E-GLAS II
Chlorinator is Pentair 320C, but I readily add liquid chlorine from the store. Big fan of the BBB approach.
Sand filter
Gray/Clear 16 Mil solar cover from "in the swim pool supplies"
GLI Storm solar cover reel, 5" tube--hornets love to make it their home.

Have a pool cleaner with second pump but it fights with the solar cover and it is LOUD from how it's coupled with small diameter hose rather than a hard line, so I don't mostly don't run it.

Record opening of the pool this year, April 29th.
Usually I get it done end of May.