I will be back in BKK in a couple of weeks. I am usually just there mid-week, but this trip I arrive on Sunday morning. Does Darling still open at 2:00PM on weekends? I know that during the week opening is normally at 4:00PM.
IMHO the best time during the weekends is around 17:00 hrs since most of the girls (if not all) will be sitting in the fishbowl by that time. Have fun.
The doors open at 1pm... by 1:30 you will find a few ladies on duty. Best selection is probably 5-ish like Pinga says, but I have never suffered any disappointment going earlier. If you go at 2, then you'll be ready for another round by 5 when the fishbowl fills up.