Over the last year and a half ive logged over 700 hours night a mixture of VFR and IFR. Ive found that not unlike some replys it varies greatly on the circumstances, for example during the wet season in Darwin it was not uncommon to fly my entire route under the gentle strobing of thunderstorms.As you can imagine even with the lights dimmed i could not percieve a great deal outside which is a bit of a nuisance when you dont have a weather radar and have to pick your way around cells for 3&half hours.Generally i would as much as possible lower my internal lights too allow the greatest possible outside accuity especially in the last half an hour before landing or with traffic around.