Plyometrics - when and how much?

Several sources recommend plyometrics to improve running efficiency. I understand that due to risk of injury it is advised to do 1 to max 2 plyo sessions per week. Typical studies include ±8 weeks before measuring the effect but that does not mean 8 weeks is optimal.

*When do I add plyometrics into my training program?

*How many weeks until plateau?

*How long does the “improvement” last?

Any advice? Coaches, experts or own experience?

Sindre