I think it all depends on what type of flowable you use since some are made to self level more than others. For instance, the Monkey Snot by Monkeyfinger needs 24 hours to cure. I’ve tried to play it before that and have had it tear out or it can still be tacky and ruins the string…plus you have to re-silicone which is just a waist of time and silicone.
I would follow the directions on the product you are using for the best results honestly. No one on here works for the manufacturer and I would trust the company that makes the product to know how best to use something they have spent money on making and testing for best results.
If you were to look up the components in liquid silicone you would see that the majority require 24 hours to cure since they technically do not actually dry but cure. The difference is that when something dries you are looking for a moisture content while curing is the process of allowing something like silicone to reach the desired moisture, viscosity and state in order to do what it was intended for. So flowable, having used it since the very first release of the YYJ hitman, I would say go with the full 24 unless the product says otherwise.