The amount that's safe depends on what it makes NA, the machining, the fasteners, the parts, and the tune. Assuming stock rods and crank and the typical 450hp NA - I'd limit it to 300. A good progressive controller could raise that a bit. The old school retard is 2° for every 100hp or for extra safety go 3°/100hp. As the system gets bigger the plugs are more important, as are the fuel and bottle pressures. 400-500 is a big hit in one shot on small tires or stock suspension.