When you become an expert on reseating tire beads because you keep blowing them off!
It depends a lot on your rim width vs tire width to be honest, and again on sidewall flexibility etc. There's no set number as every variable (engine power, braking performance, driving style, trail conditions) changes the game completely!
I'd worry about getting your rig built and getting some wheeling in before you get too concerned about ultra low pressures and beadlocks. If you're that worried start at a higher pressure and slowly work your way down until you have problems. Losing a bead is why you have a spare, and go with friends!