Approx 15-25 nm in trail is the risk area. Inside of 15 nm it would need to desecebd at 500 FPM to be 1000” lower. After 25 nm you’re outside of three minutes.
The wake isn't visible. Absent a significant wind shift contrails will give you a good indication of where the wake is.
Wake is a function of weight, wingspan, and AOA. All the larger twins have wingspans in the 210’ (65 m) range. And it drops 1000’. Been there done that. Moderate turbulence in a 777 behind a 747-400 at cruise.