[wongarsu]

For perspective, the ISS is currently orbiting at 400km altitude. Typical graveyard orbits are just beyond geostationary, so lets call it 36500km.

Orbits get somewhat emptier beyond 2500km altitude, but you'd still risk spent rocket stages or defunct satellites hitting the ISS if you don't go all the way out beyond geostationary. Maybe you can stay at lower orbits if you dock a vehicle that can provide thrust for obstacle avoidance even after the main station powers off.