There are many, many threads on this (I had the same question myself a while back, answered many, many different ways). Enter “free memory” in the search function to see a variety of suggestions and rationale for each. However, this is an answer I once got directly from the Hubitat staff:
The free memory metric in 2.3.1.142 better reflects the state of affairs, but it is different from pre-2.3.1 and even different from early 2.3.1 public releases.
This also means that the "reboot hub when free memory goes below 250" approach is no longer applicable. You can let it run all the way until the "low memory" alert pops up at 80m.”
That having been said, I used to have mine set up at 250K per @thebearmay (he has been a virtual firehose of great information and help to me), but lowered it to 175K since 2.3.1.142. I am on 2.3.2.141 (the latest firmware) and 175K gives me about 30 days between automatic reboots via RM5.1 without any noticeable slow downs in my specific use case. Of course, YMMV depending upon your set up.
Edit: link to quote:
