In BGEE, 20 Constitution is where passive HP regeneration starts. Sources discussing the game note that 20 CON gives minor regeneration, and that higher CON increases the rate further up to 25 CON.

Regen thresholds

  • 20 CON: passive regen begins.
  • 25 CON: highest normal regen rate.
  • Below 20 CON: no passive regen from Constitution alone.

Practical note

A forum example also says a character with 20 CON will regenerate, and that items or effects can push CON higher for stronger regen.

Tl;dr

You need 20 CON to get passive HP regen in BGEE.