If you do not know the time it will be difficult for you do anything but show up... I have had lunch at the RP on two occasions in the past year. I made reservations for both visits. As I recall there were several waiting w/o reservations, you can eat at the bar, but for a table I think a reservation will be necessary. I suppose the only caveat would be if you arrived early; i.e 11:30 or late, say 1:30.