It’s been documented by several fisheries bio’s in the southern states that crappie will start using cedar tree reefs within a few days of placing them. I’ve noted usage within a few months, specifically on shallow reefs, in NE reservoirs. After a late fall placement these reefs were used during all phases of the following spawn. A recruitment study revealed usage. Honestly, I would image as soon as any tree reefs are placed, fish will begin using them or evaluating them.