Do you grow under HID? I've read that BX's vermiculite holds moisture better under HIDs intense light. Apparently HP (no vermiculite) dries too quickly.
I use HP (with added perlite) under LED. It takes 2-2.5 days to dry.
I grow indoors and outdoors with HIDs, CFLs and T-5s and the good ol' sun.
The rate at which my medium dries out is dependent upon many variables.
The size of the container, the size of the plant, the light source etc.
I always add lots more vermiculite and perlite to the straight BX, as I like it to be very fluffy.
I think the most important addition to the BX is a generous amount of dolomite lime.
I have been using ProMix BX for well over a decade, and have been quite satisfied with the resulting crops.