I'm 32 weeks pregnant and have 6 cm amniotic fluid. how quickly can it go from 6 cm to 4 or 5 cm?

Depends : Amniotic fluid trails down toward end of pregnancy, however, since it is produced by the babies kidneys it is also related to moms hydration. It may stay at 6 or it might increase too. It should be rechecked in one week to make sure babies kidneys and swallowing are fine. Drink more fluid for now.

Slowly: Amniotic fluid levels usually change slowly, unless your membranes rupture--then it can be very rapid