Each operator will have their own guidance but generally it is 5-10nm horizontal separation down low and 20nm or more at altitude. On board radar is key for avoiding individual cells using the predictable and reliable precip and doppler shift returns to thread your way safely to your destination.
For terminal forecasts with 60% or greater thunderstorm coverage, additional fuel (generally 30-40 minutes) is uplifted to account for delays (ATC) or circuitous arrival paths.