In the bigger clubs (Sharks, Oase, Artemis etc.) shift change tends to be between 6pm and 8pm. That is, the girls who have been there all day are going home ( and may want a last trick to make their personal daily target) and the girls who will stay until early morning are just getting going and looking for their first room.
Early in the week tends to be much quieter, but, as you say, the weekend can be very hectic.
Personally I tended to try Wednesday through Friday for the best range of options.
Two other things to watch out for:
1) Not every day is full FKK> Each club has a "lingerie day" where the girls are not naked, so if part of your reason for going is to look at lots and lots of naked girls, choose your day wisely.
2) Some clubs, particularly Sharks, can get fairly rowdy after about 11pm, so pick your time well.