How long a steep?
It depends on exact flavours & brands.
For biscuit 4 weeks should be plenty IMO.
If no good for you after that I would think not for you (not saying no good, just not for you).
Aside:-
Remember taste is subjective & changeable to the pallet.
Ignoring DIY some will say premium, pre mixed, shop bought flavour A is like "The nectar of the greek gods themselves, smooth and flavoursome with a sweet aftertaste".
Others will say, of the exact same premium bottle of flavour "That is Horses piss, or satans vomit, I have tasted a better donkeys ass".
It may not be your mix - just that you do not like that particular flavour.