How likely are non-ECC RAM errors during a long compile?

Very low — less than a 1% chance of a single-bit flip for a multi-day compile. Soft errors from cosmic rays occur at roughly 1 bit flip per 1-2 GB of RAM per year, so with 32 GB over 3 days, it’s about a 0.3% chance. Heavy load doesn’t affect cosmic rays, but thermal stress from sustained high temps can increase hard errors. For a hobby build, stop worrying; the compiler catches most corrupted data via checksums. Memtest86+ before a long compile is smarter than buying ECC for a desktop.