how fast of a drop in pH can the plants take.

[Brigin]

Ron, when adjusting my pH level, how fast of a drop in pH can the plants take. On my pH down, the bottle suggests slight increments of .5 within a 24hr period. (but I think that is when dealing with fish). What if my pH is way out of whack and is in need of a dramatic change right now? Say it is over 7 and needs to get down to 6. How fast can this change take place. I want to do it immediately.--Please help, and thank you.--Brigin down the pH.